Engineering graphene quantum dots for enhanced ultraviolet and

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES

Article

Engineering graphene quantum dots for enhanced ultraviolet and visible light p-Si nanowire based photodetector Iuliana Mihalache, Antonio Radoi, Razvan Pascu, Cosmin Romanitan, Eugenia Vasile, and Mihaela Kusko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07667 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Engineering graphene quantum dots for enhanced ultraviolet and visible light p-Si nanowire based photodetector

Iuliana Mihalache1*, Antonio Radoi1, Razvan Pascu1, Cosmin Romanitan1,2, Eugenia Vasile1,3, Mihaela Kusko1*

1

National Institute for Research and Development in Microtechnologies (IMT Bucharest), 126A Erou Iancu Nicolae Street, 72996, Bucharest, Romania 2

3

Faculty of Physics, University of Bucharest, P.O. Box MG-11, 077125 Bucharest, Romania

Faculty of Applied Chemistry and Material Science, University Politehnica of Bucharest, No. 1-7 Gh. Polizu Street, 011061 Bucharest, Romania

Corresponding authors: *E-mail: [email protected]; [email protected]; 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

Abstract

In this work a significant improvement of the classical silicon nanowire (SiNW) based photodetector was achieved through the realization of core-shell structures using newly designed GQDPEIs via simple solution processing. The poly(ethyleneimine) assisted synthesis successfully tuned both optical and electrical properties of graphene quantum dots to fulfill the requirements for strong yellow photoluminescence emission along with large bandgap formation and the introduction of electronic states inside the bandgap. The fabrication of GQDPEIs based device was followed by systematic structural and photo-electronic investigation. Thus, GQDPEIs/SiNWs photodetector exhibited a large photocurrent to dark current ratio (Iph/Idark up to ~ 0.9 x 102 under 4 V bias) and a remarkable improvement of the external quantum efficiency values that far exceed 100%. In this frame, GQDPEIs demonstrates the ability to arbitrate both charge carriers photogeneration and transport inside heterojunction, leading to simultaneous attendance of various mechanisms: (i) efficient suppression of the dark current governed by the type I alignment in energy levels; (ii) charge photomultiplication determined by the presence of the PEI induced electron trap levels; (iii) broadband ultraviolet-to-visible downconversion effects.

Keywords Graphene quantum dots, silicon nanowires, photodetector, electron trap levels, charge photomultiplication, down conversion 2 ACS Paragon Plus Environment

Page 3 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Low-dimensional Si nanostructures (nanowires, nanorods, nanopillars, etc.) have been intensively investigated as promising candidates for optoelectronic applications. Among them, silicon nanowires (SiNWs) arrays tailored in customizable geometries, have received increasing attention due to their suitable structural, electrical, optical and thermal properties1. Classified as quasi 1D semiconductor nanostructures with high surface to volume ratio and less than 100 nm diameter, SiNWs were developed in the process of downscaling devices and revealed superior material properties compared to conventional planar silicon in a wide range of micro and nanoscale device configurations including photodetectors2, photovoltaics3, field effect transistors4, thermoelectric devices5, supercapacitors6, and ultrahigh sensitivity chemical and biological sensors7. It is notable that one-step metal-assisted electroless chemical etching of silicon substrate (MACE) approach was recently proposed as reliable room temperature wafer-scale process for fabrication of well-controlled SiNWs architectures, which presents important advantages over the standard techniques, such as vapor-liquid-solid growth, reactive ion etching, or electrochemical etching, since it preserves the same orientation, doping density, quality and crystal structure of the initial Si wafer8. Responding to the current demand of developing high efficiency photodetectors, heterojunctions based on Si nanowires have been gaining special interest in the research community due to certain advantages in terms of low cost and ease of fabrication and the potential of being scalable and integrated with conventional fabrication lines9. In the recent years, SiNWs-based photodetectors concepts as band gap engineered devices with

modulated interface geometry have been demonstrated using various semiconductor thin films of III−V or II−VI semiconductor QDs (CdS10,11, CdTe12, ZnTe13, etc.), oxides (ZnO14, TiO215, etc.) and even polymeric acids16. Compared to conventional p-n junction, radial core-shell heterojunction photodiode have the advantage of enhanced light absorption and spectral response, and furthermore of more effective charge carrier separation in the radial direction, where the carrier collection distance is smaller or comparable to the minority carrier diffusion length17. As an alternative, carbon-based materials/Si NWs heterojunction takes advantage of the active photogeneration sites, a percolating network for charge transport, and transparent electrode for illumination and charge carriers collection. Consequently, they might respond to prerequisite criteria for improvement of the SiNWs based photodetector, such as suppression of carrier recombination, or enhancement of light absorption and carrier transportation18. Graphene quantum dots (GQDs) are known as emerging counterparts of traditional semiconductor QDs, often outperforming common chromophores in terms of tunable physical and chemical properties, ease of functionalization, photo-stability and nontoxicity. Besides, parameters such as longer carrier 3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 32

lifetime, charge-carrier mobility and both n- or p-type conductivity can be optimized during the synthesis process. GQDs have also some important features related to device integration like water solubility, fast and reliable methods for extraction into organic solvents and solution-processability which allows versatility with a great variety of substrates. Their properties were explored beyond usual chromophore targeted application so that energy storage and generation19

20 21 22

, memory devices23, photocatalysis24 and light-emitting diodes25

application have been reported demonstrating superior electrical conductivity and charge transfer capabilities. The light absorption/emission properties of GQDs (i.e. absorption up to ~ 6 eV and blue-luminescence) make this material particularly promising for high gain photodetection in a broad wavelength range, significant photoconductive response being demonstrated firstly in ultraviolet (UV)23 and deep UV26 range, and recently, visible-blind UV photodetector has been fabricated using GQDs sensitized ZnO nanorods27, 28. A photodetector based on a single graphene nanodots layer sandwich between graphene sheets reached around 80% quantum efficiency throughout visible range due to effective tunneling of charge carriers through the energy states in GQDs29. Furthermore, studies showed that the photoelectrochemical performance of Cu2S nanowire arrays30 and highly orientated SiNWs electrodes31 was improved due to charge separation and energy-down shift effects induced by the presence carbon quantum dots (CQDs). Similarly, significant enhancement of photoconductive and photovoltaic properties of p-type CuAlO2/n-type ZnO photoelectric bilayer films32 was obtained when added CQDs contributed to the separation of photogenerated charge carriers and luminescence upconversion effect. Additionally, Xie et al. study showed that core-shell heterojunction formed between the n-type SiNWs array and CQDs synthesized by electrochemical etching method perform as solar cell with up to 9.1% power conversion efficiency and self-powered photodetector33. Starting from our previous findings regarding the ability to manipulate GQDs band gap21 and also to control the generation of supplementary energy trap levels inside band gap by adding poly(ethylene glycol) during a simple one-step microwave assisted synthesis23, this paper exploits the effect of poly(ethyleneimine) (PEI) on the GQDs electrical characteristics, which is a polymer with large charge density, capable to act indirectly as electron donator34,35. Accordingly, the fabrication process was tuned to achieve the most appropriate material in terms of absorption and emission properties or band gap engineering criteria. We investigated the structures obtained by coating semiconducting vertically aligned p-type SiNWs arrays, fabricated using MACE approach, with a thin layer of GQDPEIs via solution processing, leading to the formation of a radial heterojunction along the whole nanowire length. Following the fabrication, micro-structural and photo-electronic characteristics of the core-shell junctions have been systematically investigated and discussed. Herein, we further demonstrated a highly efficient type I alignment GQDPEIs/SiNWs photodetector by virtue of the physical mechanisms fostered by GQDPEIs, and revealed by our thorough optical and electrical investigations. Thanks to the unique electrical properties and band structures of GQDPEIs, the quantum dots 4 ACS Paragon Plus Environment

Page 5 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


layer could serve as hole blocking layer in the device. The traps present in quantum dots play a major role in photocurrent generation as they stimulate the carrier photomultiplication process and consequently endows the heterojunction with considerably broadband photosensing. On the other hand, GQDPEIs designed to boost the optical detection domain of SiNWs counterpart lead to the appearance of downconversion phenomena. Further studies are performed to clarify the influence of energy transfer enhancement effect on quantum efficiency of GQDPEIs/SiNWs photodetector and the non-radiative energy transfer contribution is investigated.

2. Experimental section 2.1. Preparation of SiNWs and GQDPEIs. Acetone, H2O2 (31%), H2SO4 (96%), HF (50%) and HNO3 (69%), all VLSI grade were from BASF, Germany. Ethanol, AgNO3, D (+) glucosamine hydrochloride and poly(ethyleneimine) solution (50 % w/v) were acquired from Sigma, Germany. Extran® solution was from Merck Millipore, Germany. Commercial ptype Si (100) wafers with boron doping of 2 × 1016/cm3 and resistivity of 3-5 Ω.cm were cut into 2 x 2 cm pieces then sonicated in ethanol, acetone and (5 % v/v) Extran® solution (during 15 min.) and ultimately cleaned in Piranha solution (3:1 v/v H2SO4 and H2O2) for 30 min. at room temperature; between each step the samples were rinsed with deionized water (DI). Vertical SiNW arrays were fabricated using metal-assisted chemical etching (MACE) technique. Substrates were immersed in 4.8 M HF and 0.02 M AgNO3 aqueous solution for 1 min. at room temperature achieving an Ag nanoparticle mesh film on the Si surface, then rinsed thoroughly with DI water to remove the extra Ag+ ions. In the second step, Ag-deposited samples were etched in 4.8 M HF and 0.1 M H2O2 solution mixture during 5, 10, 15, 20 minutes, in the dark, at room temperature, then washed with DI water. At this point, SiNWs were formed and samples were dipped into concentrated HNO3 (65%) for 30 minutes in order to remove any residual catalyst, the resulting black surface samples are copiously washed with DI water and dried under N2 flow. The synthesis of graphene quantum dots (GQDs) was achieved by one-step microwave-assisted hydrothermal method, using a Parr Acid Digestion Vessel with PTFE cup and a commercial microwave oven. Briefly, 1.35 g of poly(ethyleneimine) (PEI) were homogenized with DI water (3.3 mL) and thereafter 2.30 g of glucosamine were added and solubilized during 5 minutes at 25 kHz in an ultrasounds bath (Elma, Germany). The reaction vial was held at 40 °C during 1 hour and the entire solution was poured in the PTFE cup, sealed in the microwave acid digestion vessel and introduced for 7 s in the microwave oven at an applied power of 700 W. The reaction vessel was allowed to cool down, at 4 °C for 30 minutes, and then the obtained solution containing GQDPEIs was ready to be used for the envisaged experiments. During the above described protocol, the prepared solution changed its color, changing from transparent/pale yellow (after sonication) to dark yellow 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 32

(after the incubation at 40 °C) and finally reaching a brownish coloration, indicating the formation of PEI based GQDs – GQDPEIs. 2.2. Fabrication of GQDPEIs/SiNWs heterostructure In our optimized procedure, substrates were first exposed to UV/ozone surface treatment for 30 min at 90°C to improve the hydrophilicity of the surface, then cooled down to room temperature (RT). A controlled thin and uniform coating was achieved for the entire Si nanowires surface by spin coating a GQDPEIs aqueous solution of previously determined volume and concentration onto the top of the substrate. This coating step was repeated for up to twelve times followed by air-drying at RT. An indium tin oxide (ITO) layer of ~ 200 nm thickness was deposited by DC magnetron sputtering and used as typical transparent front contact. The last step consists of carefully removing the backside silicon nanowires by etching the substrate with a mixture of hydrofluoric acid, nitric acid and acetic acid (18 mL HF (50%) + 57 mL HNO3 (69%) + 25 mL CH3COOH (90%)) then cleaned and dried under N2 flow prior to the deposition of 200 nm thickness Al layer by thermal evaporation in order to realize the bottom ohmic electrode. 2.3. Characterization and measurements. The transmission electron microscopy (TEM) measurements were performed on FEI Tecnai G2-F30 STwin field-emission gun scanning transmission electron microscope (FEG STEM) at an accelerating voltage of 300 kV. The XRD measurements were performed using a 9 kW rotating anode Rigaku SmartLab thin films diffraction system with a parallel beam setup. Absorption spectrum was investigated using a Hitachi U-0080D spectrophotometer. Photoluminescence measurements were performed on Edinburgh Instrument F920 spectrometer equipped with 450 W Xenon lamp excitation source. Moreover, the monocromator of F920 spectrometer was incremented over UV-Vis spectrum to generate the wavelength dependent responsivity, while the intensity of incident monochromatic light (mW/cm2) was measured with an optical power meter. Autolab/PGSTAT302N potentiostat/galvanostat and a three electrode set-up (Pt disc as working electrode, Pt wire as counter electrode and RE-7 non-aqueous reference electrode (BAS Inc., Japan)) in an acetonitrile containing 0.1 M tetrabutylammonium perchlorate (TBAP) were used during electrochemical experiments. DC measurements were performed with the semiconductor characterization system Keithley system 4200-SCS coupled with an EP6/Suss MicroTec microprober, connected to a dark Faraday cage, under dark and light conditions. Samples were illuminated using a LED solar simulator, class ABA (model LSH 7320, Newport, USA). The light intensity of the lamp was controlled by change of the current across the lamp and was further adjusted with neutral density filter. The impedance measurements were carried out both in dark and under constant light illumination, with frequency ranging from 1 kHz to 1 MHz. 6 ACS Paragon Plus Environment

Page 7 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


All the optical and electrical measurements were carried out at room temperature.

3. Results and discussions Fig. 1 (a) shows the HR-TEM image of GQDPEIs with a narrow size distribution width from 3 nm to 6 nm and an average particle diameter of ~ 4 nm according to statistical particle size distribution – Fig. 1 (b). The crystallinity is also evidenced and the associated lattice parameters (d-spacing) are 0.31 nm and 0.21 nm corresponding to the (002) and (100) planes of graphite36 (see also Fig. 2 (c) – right side). GQDPEIs samples were investigated by absorption, emission, and luminescence quantum yield measurements at room temperature. Fig. 1 (c) shows the absorption spectrum of GQDPEIs ranging from 190 to 650 nm, beyond which they are transparent to the visible light. The optical absorption spectrum displays two sharp distinct peaks situated well within the UV range and an absorption tail extending into visible region due to the high density surface states, allowing the excitation with spectrally wide light source. The intense absorbance maximum at 200 nm (6.2 eV) and 275 nm (4.5 eV) wavelength originate from graphene core and were assigned to π-π* transitions of C=C bounds. Such strong transitions suggest the presence of a high density of delocalized πelectrons which are mainly responsible for electrical conduction in graphene-based materials. The presence of a broad absorption band located after 300 nm wavelength corresponds to the observed luminescence so it could be attributed with both σ-π (HOMO-LUMO molecular orbitals) and n-π* electronic transitions in surface states induced by surface chemical modification of carbon backbone by various functional groups. It can be observed that the excitonic absorbance features of the GQDPEIs casted as a film are almost identical, featuring only a slight redshift. The optical energy gap at 2.83 eV is consistent with HOMO-LUMO results obtained by electrochemical analyses. Hence, the energy levels of the synthesized GQDPEIs were determined using cyclic voltammetry measurements, which allow solving the absolute positions of the band potentials, relative to the vacuum scale (Fig. S1, Supporting Information). Therefore, we used the potential values corresponding to oxidation and reduction onsets in order to determine the highest occupied molecular-orbital (i.e. EHOMO energy level), lowest unoccupied molecular-orbital (i.e. ELUMO energy level), and to find the defect/surface state levels37. The EHOMO, ELUMO values calculated from oxidation and reduction peaks onsets are − 3.28 eV and − 6.02 eV, respectively. Besides, the appearance of additional electrochemical peaks reveals the presence of intermediate energy levels induced by polymer attachment, located inside of the band gap. Thus, in comparison with self-passivated GQDs, increasing the polymer quantity during the synthesis determines a significant increase of the area of the voltammograme in the LUMO corresponding region that manifests as adjacent peaks merging and fine tuning alignment of the energy levels.

7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 32

Typically, GQDs exhibit broad excitation dependent photoluminescence emission bandwidths as their main optical characteristic. Starting with self-passivated GQDs from glucosamine pyrolysis which exhibit descending intensity PL emission bands with weak emission signal recorded after 480 nm excitation23, we designed the synthesis of a new GQDPEIs material displaying a yellow PL emission that can be effectively collected by the SiNWs device. Moving GQDs emission wavelengths toward the red end of the visible spectrum is a current challenge despite the increasing number of synthetic routes developed in recent years. In the matter of polyamine-functionalized CQDs38, our synthesis technique was develop to meet the device requirements such as larger bandgap formation, the presence of electronic states inside the bandgap and the redshift of the strongest emission peak, which occurred at 575 nm. The photoluminescence decay profile shown in Fig. 1 (e), was fitted to a triple exponential function resulting three decay components: two fast decay constants, one of them less than 1 ns (0.69 ns) and the second one of 2.71 ns, attributed to exciton recombination in the internal core states, and a slow decay constant of 8.7 ns, attributed to localized surface defect states39. The absolute quantum yield of GQDPEIs was determined to be 7% at 480 nm excitation wavelength. Also, they demonstrated superior color, brightness and photostability that remain stable for several months. The novel photodetector device was build starting from a platform of vertically standing p-type SiNWs arrays with ~ 8.8 µm length and ~ 60 nm ± 5 nm diameter nanowires densely distributed (Fig. 2 (a) - left inset). The XRD analyses (Fig. S2, Supporting Information) confirm on the one hand that the MACE process did not destroy or modify the quality of crystallinity of SiNWs, consequently, Si (100) peak related to starting silicon wafer is clearly observable, being consistent with previous reported results8. On the other hand, there are no additional peaks associated with residual catalyst nanoparticles. The presence of GQDPEI layer coating the silicon nanowires is evidenced by a new peak positioned at 28.3°, which is generally the signature of GQDs, being close to the lattice spacing of d002=0.31 nm revealed also by the high resolution TEM analysis presented in Fig. 2 (c). Both the SiNWs array geometries and the GQDPEIs colloidal solution concentrations were selected using successive optimization steps based on the photodetection performance analysis (Fig. S3, Supporting Information). Accordingly, 4 types of SiNW substrates, with nanowire lengths ranging from 3 to 13 µm, were spin-coated with 1, 2, and 3 layers of GQDPEIs, and the most promising structure was the one obtained when the etching time was set at 10 minutes. Similarly, 7 serial dilutions of GQDPEIs colloidal dispersion obtained from stock solution were tested in order to select the optimal concentration. After the deposition of GQDPEIs, it can be observed that the pattern is maintained and nanowires remain in separate condition (Fig. 2 (a) - right inset). After ITO deposition, the initial surface morphology changes and nanowires self-assembles into percolating networks of nanowalls allowing new conductive paths for carrier transportation (Fig. 2 (a) - main image). The amalgamation was obtained while adjusting the ITO sputtering deposition parameters in order to achieve 8 ACS Paragon Plus Environment

Page 9 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


complete coverage along the length of the nanowires. During the final stage of deposition, the appearance of lateral coalescence40 causes adjacent nanowires to bend or slant with a certain degree, as can be clearly observed in the magnified SEM images – Fig. 2 (b), leading to the creation of local clustering and connections between the neighboring clusters that provide higher current conduction. The surface profiles of the core-shell heterostructure generally replicates the surface morphologies of the nanowire with slightly larger and roundshaped tops (Fig. 2 (b) – inset). A bilayered shell with less then about 10 nm thickness made of a continuous film of closely packed GQDPEIs and a transparent ITO film, was achieved to envelop the Si nanowire core, evidenced by the parallel lattice fringes measured as 0.19 nm, which is close to the (220) spacing planes, as revealed by the high resolution TEM image of the transversal cross-section of the heterojunction – Fig. 2 (c). Energy-dispersive X-ray spectroscopy (EDXS) analysis shown in Fig. 2 (d) further confirmed both components have penetrate down to the root of nanowires and the surface was covered entirely. The electrical characteristics of the ITO/nL-GQDPEIs/SiNWs/Al structures were measured at room temperature, by applying voltage at the top contact (ITO), whereas the bottom contact (Al) was grounded. Consequently, the forward region of the current – voltage (I-V) characteristics appears at negative voltages, and the reverse one appears at positive voltages. The semilogaritmic plots of measured current vs voltage are presented in Fig. 3 (a), and a Schottky diode-like behavior can be observed, both for ITO/SiNWs/Al (REF) and ITO/GQDPEIs/SiNWs/Al (3L, 6L, 9L, 12L, respectively) samples. It is noteworthy that the presence of GQDPEIs and formation of the GQDPEIs /SiNWs core-shell heterojunctions also have a beneficial effect on rectifying characteristics, and their effects on the heterostructure parameters such as ideality factor, rectifying current ratio, barrier height, and saturation current obtained from the dark current analyses are presented in Fig. S4 (Supporting Information). These Schottky parameters have been calculated using the thermionic emission model, expressed by the following relation:

= − 1

(1)

where kB was the Boltzmann’s constant, I0 was the reverse saturation current, V was the applied voltage, n was the junction ideality factor, and T was the temperature in Kelvin. The reverse saturation current I0 in eq. (1) is given by the following relation:

= ∗ −

(2)

where ΦB is the effective potential barrier, A is the device area, and A∗ is Richardson’s constant41. The values of the ideality factor, determined from the slopes of the exponential region, were found to range between 4.41 and 2.14 with increasing the quantity of loaded GQDPEIs, leading to a shell more uniformly distributed from layer to layer. The high values of the ideality factor, n, are specific for a non-ideal behavior, 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 32

showing the existence of some inhomogeneity on the contact, doping non-uniformity with increasing of GQDPEIs layers and the presence of interface traps. These electrical defects are generally attributed to the rough surface for etched Si nanowires11,42, but also may be determined by the multi-interface nano-heterojunction formed in the GQDPEIs/SiNWs nano-assembly. However the n values obtained are lower compared to other ones reported for similar quantum dots silicon nanowires heterostructures, such as CdS/SiNWs10, demonstrating the formation of high-quality heterojunction between GQDPEIs and SiNWs. A similar trend was observed for the forward-to-reverse rectifying ratio (IF/IR) at ± 4V which systematically increases more than 12 times, from ~ 80 which is the value corresponding to the structure without GQDPEIs (reference), to ~ 980 obtained after 12 layers GQDPEIs deposition. Because the interfaces are not atomically flat and often the real Schottky contacts exhibit a lot of inhomogeneities, especially when the electrical characterization is done at room temperature, a variation in Schottky barrier height (Φb) can be observed with the number of GQDPEIs layers. Modification of the interfacial chemistry of SiNWs by GQDPEIs is revealed with the enhancement of Φb, a plateau is visible when 3 to 9 GQDPEIs layers are deposited and it is followed by a second increase at 12 layers. According to eq. (2), an opposite behavior is obtained for the saturation current (Is), which diminished with increasing number of GQDPEIs layers. When the experimental heterostructures are illuminated with broadband light of 50 mW/cm2 intensity, it is evident that GQDPEIs play an important role for superior photoconductive behavior. The photocurrent increases rapidly as the bias increases to 4V whereas above this voltage, there is only a slow increase without reaching the complete saturation at 10V reverse bias. The photoresponse behavior is characterized by low open circuit voltage (Voc), low short circuit current, but high photocurrent values. If the pristine structure presents a Voc of only few mV, along with GQDPEIs addition the Voc significantly increases from 185 mV for 3L- GQDPEIs, to 218 mV for 12L- GQDPEIs, proving the heterojunction formation. The obtained values are low for solar cell applications, but the optimization of heterostructure fabrication processes can be achieved, including different functionalization of SiNWs prior GQDPEIs deposition43. Regarding photodetection, the GQDPEIs - sensitized structures show higher photocurrent (3 to 5 times) compared to reference SiNWs sample, the maximum enhancement being observed for 6L- GQDPEIs/SiNWs. The light-to-dark current ratio performance metric is plotted as function of bias voltage in Fig. 3 (b), which improves with increasing applied voltage from 0 to 4 V before reaching saturation regime. It is notable that the enhancement of photocurrent is correlated with the strong suppression of the dark current, leading finally to a significant improvement of the Ilight/Idark ratio, from a ratio of 1.4÷1.5 calculated for the entire range of positive voltages for reference sample to a maximum ratio of ~ 90 obtained for 6L-GQDPEIs/SiNWs at a bias of 4 V. The I−V measurements clearly demonstrate that the best device performance can be obtained when 6 layers of GQDPEIs were deposited on SiNWs. Our estimated sensitivity is better than the reported results for n-TiO2 10 ACS Paragon Plus Environment

Page 11 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


capped p-SiNWs heterojunction photodiodes (i.e. ~ 40 at 4 V) 44. The decrease of photoresponse properties above 6L-GQDPEIs might be attributed to aggregation of quantum dots and formation of thick, compact film of GQDPEIs on SiNWs surface, blocking the photons to reach the GQDPEI/SiNW junction while recombination of the photoexcited charge carriers taking place at the boundary of the quantum dots45. The photoresponse properties of heterostructures were evaluated using varied light intensities under 4 V reverse bias, the inflection point in Ilight/Idark plots, and additionally, under lower (1 V) and higher (10 V) voltage biases, respectively, in order to characterize the device photoconductive gain directly. The transient photocurrents obtained at 4 V bias when illumination with light intensities ranging 5 to 50 mW/cm2 is turned on and off alternatively are shown in Fig. 3 (c), and it can be observed that the current sharply increases/decreases. Furthermore, the photocurrent value of GQDPEIs/SiNWs junction can reach as high as 2 mA under low power illumination of 5 mW/cm2, which is 20 times that of SiNWs device. The photo to dark current ratio is directly related to the incident power – Fig. 3 (d) – and, obviously, the presence of GQDPEIs layers lead to significant enhancement of the photoresponse, the Iph/Idark ratio reaching at least one order higher values than those obtained for the pristine structure, where photocurrent (Iph) is defined as difference between current in the dark (Idark) and under illumination (Ilight). Moreover, while the reference presents a slight decrease of the response when the incident power exceeds 40 mW/cm2, starting with six layers of quantum dots the increasing response follows the incident power. It is notable that the largest increase takes place at low illuminations, which is highly beneficial for the photosensing ability. The photo to dark current ratio obtained when the structures are biased at 1 V and 10 V, respectively, are presented in Fig. S5 (Supporting Information). To further quantify and clearly compare the performance of the heterojunction photodetectors, we estimated the responsivity (R), external quantum efficiency (EQE) and detectivity (D) of the devices for incident light wavelengths ranging between 340 and 880 nm. These parameters were calculated using the following equations46:

= =

'=

((*)

! "#$%&

-./ , #$%& 0

121 (%) = 100 ×

(3) (4)

78×((*)

(5)

*

where Iph is photocurrent, Popt incident light power, q the electron charge, S the active contact area, and λ the incident light wavelength. Fig. 4 presents the responsivity (R), EQE and detectivity (D) plotted as a function of wavelength for reference and hybrid heterostructure photodetectors evaluated under 4V reverse bias. These results indicate that 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 32

GQDPEIs loading generate significant improvement in conversion efficiency over the ultraviolet and visible detection range while the most pronounce spectral feature of SiNWs reference was maintained in the GQDs based heterostructures. The responsivity (R) peak located near 750 nm due to high absorption coefficients of ptype Si nanowires at longer wavelengths11 reaches a maximum value of ~25 A/W with corresponding detectivity of ~ 9 x 1012 Jones for 6L-GQDPEIs/SiNWs structure. Interestingly, the largest increase of responsivity (R) was achieved in the wavelength region below 550 nm. The extension of UV detection sensitivity clearly confirms the contribution of graphene quantum dots optical properties leading to maximized EQE enhancement as shown in Fig. 4 (d). It is notable that the values obtained for each of these three parameters are remarkable, with at least one order of magnitude larger than the reference structure whose values are comparable to previously reported ITO/SiNWs photodetectors47. But, the outstanding results are obvious the remarkable EQE values obtained for GQDPEIs/p-SiNWs photodetectors that become one order of magnitude higher, above 100%, regardless of the number of quantum dots layers, reaching 4200% for 6L-GQDPEIs/SiNWs structure at 4V reverse bias. A similar behavior can be observed biasing the structures at lower (1V) and higher (10V) voltage biases, respectively, and the dependence of all parameters on the bias voltage is plotted in Fig. S6 (Supporting Information). Accordingly, whereas at 1V bias the EQE varies between 387% and 818% depending on the number of GQDPEI layers, at 10 V bias, it becomes one order of magnitude higher for each structure, reaching 8150% when six layers of GQDPEI are coating the SiNWs. These extremely high values obtained for a standard quantum dots – silicon nanowires core-shell heterostructure, which are approximately 2 orders of magnitude higher than those reported for CdTe/SiNWs heterostructures12 and at least 3 fold higher than those reported for CdS/SiNWs heterostructures at 1V bias11, are generally attributed to the trap induced carrier injection on nanowires, leading to generation of extra charges in the device48. A general view of the best architecture response, ITO/6L- GQDPEIs/p-SiNWs/Al, is presented in Fig. S7 (Supporting Information). Clearly, both the responsivity, the detectivity and the external quantum efficiency increase as function of the operation voltage, more rapidly for low biases, following a saturation tendency around 10 V - Fig. S7 (a-c). It can be observed that the calculated parameters follows the light current trend Fig. 3 (b) - the values obtained at 4 V bias being at least five fold larger than those obtained at 1V bias, while the last values, at 10 V bias, only slightly increase, the percentage varying between 10% in the case of detectivity and 70% in the case of EQE. The highest photoresponse values were found to be R = 40.6 A/W, D = 11.9 x 1012 cm Hz1/2/W, and EQE = 8150 %, respectively (Fig. S7 (a-c) - Supporting Information), at 10V reverse bias. To compare the device performances with other similar structures employing SiNWs or quantum dots, Table S1 (Supporting Information) summarizes key metrics including R, D, EQE, and rise/fall times, respectively. It is evident that our device shows substantially increased values of the responsivity, detectivity 12 ACS Paragon Plus Environment

Page 13 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 3940 41 42 43 44 45 46 47 48 49 5051 52 53 54 55 56 57 58 59 60


and external quantum efficiency, which are at least 5 times higher than the previously reported ones, even at low voltages. Concerning the response/recovery times, they are only tens of milliseconds at 4 V operation voltage for a light intensity of 5 µW/cm2, similar with those obtained under the lights-on, lights-off conditions. They were estimated to be the time taken to reach 90% of response after light switch on, and the time taken by the samples to recover to 10% of the baseline current, after light switch off. When the device biased at 4 V was exposed to different white sunlight intensities, a strong dependence on the incident irradiation was observed (Fig. S7 (d), Supporting Information). Two domains of variation can be identified: the first one, at low light intensities, where the photocurrent shows a good linearity over more than 3 orders of magnitude that is held for intensities varying from 5 µW/cm2 up to 7 mW/cm2, followed by an attenuated increase after 7 mW/cm2. The dependence can be simply fitted by a power law Iph ∝ Pθ, where the exponent takes the value of 0.82 for the first region, and 0.27 for the second region, respectively. A similar behavior has been reported for nanowires/nanostructure based photodetectors49 50 51, and it can be caused by the phenomena that control the photocurrent: while at low intensities the efficiency of the charge carriers photogeneration is proportional to the absorbed photon flux, at higher intensities, the photocurrent is limited by the incidence of the photogenerated charges recombination and also by the saturation of the sensitizing energy levels, when the current mainly arises from long lifetime electrons. The unveiling of the phenomena leading to significant improvement of the photoresponse was realized starting with correlation of the electrical results to the energy levels alignment between the components of the hybrid ITO/nL- GQDPEIs/p-SiNWs/Al system against the standard ITO/SiNWs/Al system. Accordingly, Fig. 5 shows the schematic diagram of the energy levels relative to the vacuum level, according to the Anderson rule for the electron affinities52. In the case of the reference structure, without the quantum dot layer covering the Si nanowires, the I-V characteristics revealed that the reverse bias current has a more pronounced exponential variation with applied voltage, corresponding to a dual back-to-back Schottky diodes behavior, in accordance with the work function differences between the Fermi levels of electrode contacts (ITO and Al) and that of the p-type Si evidenced in Fig. 5 (a). Upon light irradiation (Fig. 5 (b)), the carriers generated in Si might recombine both at top and bottom electrodes, leading to a slight increase of photocurrent. In the case of the GQDPEI based structures, the bandgap of the silicon is completely contained in the bandgap of the quantum dots and, as a consequence, when GQDPEIs and SiNWs contact, a type I heterojunction band alignment results, with a large barrier potential created at the interface. Consequently, the charge transport towards electrodes is hindered, consistent with the dark I-V characteristics. Moreover, the higher barrier height of GQDPEIs/SiNWs heterojunction in comparison with the initial ITO/SiNWs Schottky junction should lead, also, to larger Voc. As can be observed in Fig. 5 (c), under the reverse bias, the large ∆Ev offset (0.85 eV) in the 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 32

GQDPEIs/SiNWs device could represent a large barrier for hole transport from SiNWs to the ITO electrode, leading to minimized recombination on the ITO side by confining the holes in silicon53. Consequently, the GQDPEIs film could act as a hole blocking layer in the GQDPEIs/SiNWs device. Under light irradiation, Fig. 5 (d), when the GQDPEI layer is sufficiently thin, electron-hole pairs are photogenerated in both SiNW and GQDPEI sides, as well as within the space charge region. Subsequent, the charges separated through the built-in electric field can be collected by the metallic contacts, the electrons sweeping to the GQDPEI side towards the front contact, while the holes cross the silicon towards the back contact. Apparently, the electron transport is limited by the large Ec – LUMO offset at the interface GQDPEIs/SiNWs. But, in this case, the key role is played by the presence of electron trap energy levels inside of the GQDPEI gap. Thus, the GQDPEIs’ ability to trap electrons under illumination leads to photo-induced dropping of the hole injection barrier and consequently fosters the secondary carrier injection under reverse bias, boosting the EQE54. The mediator role played by the GQDPEI traps in the photoconduction is also confirmed by the observed saturation of the photoresponse, because at higher illumination intensities the number of electron traps present becomes insufficient for the photogenerated carriers55. Additionally, the carriers trapping mechanism on the intermediate energy levels of the GQDPEIs extends electron lifetime and prevents the premature recombination of electrons and holes. Therefore, the GQDPEI layers is not only a hole-blocking layer, but also acts as a photonaddressable optoelectronic valve56 that controls hole injection under reverse bias. As for the GQDPEI thickness dependence of the photovoltaic performances, a certain thickness of GQDPEI layer may have an actual lightblocking effect, which causes significant optical loss, observable when more than six layers of quantum dots are loaded on SiNWs substrate. Towards clarifying the charge distribution near the junction interface and the in-depth understanding of all associated electrical properties, capacitance – voltage and impedance spectroscopy analyses were further performed. The capacitance - voltage (C-V) characteristics were measured for different applied frequencies, ranging from 5 kHz to 200 kHz (Fig. S8, Supporting Information). The steepness of C-V plots, as well as the value of capacitance, strongly depend on the applied frequency, increasing with the decrease in frequency, due to the time dependent response of interface states57. Such frequency dispersion confirms the presence of a significant amount of traps at the hetero-junction. The C-V plots obtained for ITO/nL-GQDPEIs/p-SiNWs/Al heterostructures (where n = 3÷12 layers) at the applied frequency of 5 kHz are shown in Fig. 6 (a), a shift to the negative bias voltages being visible after the GQDPEI layers deposition. From C-2-V plots (inset of Fig. 6 (a)), after a linear fit on depletion region which shows an approximately linear bias relationship, we have determined the carrier concentration and built-in potential from the slope and intercept on x-axis of the linear fit via MottSchottky analyses:


Page 15 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


; " =

( < " )

(6)

=- >? >% @

where Vbi is the built-in potential, V is the applied voltage, A is the contact area, ε is the relative permittivity, ε0 is the vacuum permittivity, and N is the carrier concentration in the depletion region. An interesting dependence of these parameters of the GQDPEIs/SiNWs heterojunction on the number of layers is revealed in Fig. 6 (b). Hence, starting from the lowest built-in voltage and carrier concentration characteristic for the reference structure, the incremental addition of GQDPEIs of up to 6 layers determines an increase of these parameters, followed by their reduction when supplementary layers are deposited. This behavior corresponds to the heterostructures photodetection performances, and clearly confirms that the current increasing rate is determined by the increased carrier concentration58. Moreover, the value of Vbi reflects the band bending degree near the Si surface, and it controls the improvement of Voc59. The depletion width (Wd) can be also deduced from the experimental C-V measurements, and at an applied bias V, it is given by the formula: AB = ,

>? >% ( < " )

(7)

@

The apparent doping density profiles N(Wd) obtained from the C-2-V plots are presented in Fig. 6 (c). The narrow depletion region, obviously occurring in 6L-GQDPEIs/SiNWs structure, indicates a low barrier for hole transport, and determines the decrease of the photogenerated carriers recombination, leading to high photocurrent60. The carrier concentration profiles as function of the applied voltage, highlighted for the depletion region, where the capacitance is changing linearly with voltage, are showed in Fig. S9 (Supporting Information). Variation of the capacitance as function of frequency at 0V bias (20 mV AC rms superimposed) is presented in Fig. 6 (d), confirming the results obtained from the standard C-V plots. Fig. S10 (Supporting Information) shows plots of carrier concentration (N) versus frequency and the built-in potential (Vbi) versus frequency obtained from the slope of the linear plot of C-2 – V curves. As can be seen in Fig. S10 (a), N decreases with increasing frequency, because only at low frequencies all the interface states can follow the AC applied signal, and the corresponding interface state capacitance appears directly in parallel with the depletion capacitance, resulting implicitly a higher total value of the capacitance for Schottky diodes, confirming the C-V behavior61. A supplementary confirmation of the presence of the photo-generated carriers in the space charge region is accomplished by the transient photocapacitance analysis at 0.1 V bias, 5 kHz frequency using different light intensities – Fig. 6 (e). This scenario supports both the dark current suppression and the remarkable improvement of photocurrent collection efficiency. Definitely, the evaluation of optical contribution of GQDPEIs on superior photodetection performance was also mandatory. Remarkable EQE enhancement, defined as the EQE of the devices with GQDPEIs divided by that of the reference device, is shown in Fig. 4 (d) and Fig. S7 (Supporting Information). Distinct from the 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 32

enhancement by more than one order of magnitude achieved over visible wavelengths region due to increased electrons and holes collection efficiency under the effect of the electrical field, there is a growing efficiency improvement in the short-wavelength region with a peak around 500 nm, reaching as high as 33% at 4V and 46% at 10V for 12 layers. This clearly indicates that GQDPEIs, as efficient photon absorbers and large Stokes shift emitters, mediate the downconversion phenomenon. Absorbance and photoluminescence emission (PL) and photoluminescence excitation (PLE) spectroscopy measurements were performed to demonstrate how optical properties of GQDPEIs directly affect the performance of the device. The absorbance recorded for SiNWs substrate with and without GQDPEIs n-layers, is higher for SiNWs reference and very slightly decreases when more quantum dot layers where added, as it can be seen in Fig. 7 (a). The absorption in the 400 – 800 nm wavelength region decreases from 3 to 12 layers by 1 to up to 6.4 % at 700 nm while in 300 - 400 nm wavelength region remains roughly constant after the first three layers were added. In the UV region the GQDPEIs absorption coefficient actively compensates the reduction in absorbance of SiNWs which is expected while increasing the thickness of the GQDPEIs layer. Although absorption on 6L- GQDPEIs/SiNWs structure decreases by up to 3% in the visible region, the contribution of down-shifted photons remarkably enhances the external efficiency of the 350-600 nm region in the ITO/6LGQDPEIs/SiNWs/Al device by 20% (at 4V) up to 38% (at 10V). The schematic representation in Fig. 7 (b) serves the purpose to illustrate the energy downconversion capability of solid state GQDPEIs, which can effectively absorb UV and visible light at any given excitation wavelength between 300-650 nm and emit yellow-light photoluminescence at 580 nm where the SiNWs, are able to perform efficiently the photocurrent conversion. To experimentally demonstrate energy downconversion effect of graphene quantum dots, the behavior of 6L-GQDPEIs films spin coated on planar Si and nanowires was analyzed and the results are shown in Fig. 7 (c) and Fig. 7 (c) - inset, respectively. In both cases, solid state GQDPEIs exhibit a unique PL excitationindependent broadband with emission peaks situated at 580 nm for excitation wavelengths between 320-520 nm, however a strong PL intensity quenching appears in the case of GQDPEIs /SiNWs structure (Fig. 7 (c) inset) which correlates well with the increase of EQE in this active region. Interestingly, the photoluminescence excitation spectral shape suffers a dramatical modification compared to the GQDPEIs film solely, which arises from the way light is captured and absorbed inside nanowire arrays with subwavelength spacing, relative to the planar surface. The emission intensities in GQDPEIs/SiNWs PLE at 580 nm are directly related to an enhancement in the absorption process, connected to the enhanced absorption in SiNWs array. By multiplying the normalized profiles of solid state GQDPEIs PLE and SiNWs absorption data we obtained a curve that matches the experimental behavior of GQDPEIs/SiNWs PLE between 420-520 nm wavelengths (Fig. S11 Supporting Information). The results demonstrate that incident wavelengths that couples to the leaky waveguide 16 ACS Paragon Plus Environment

Page 17 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


modes inducing the absorbance resonance in the NWs arrays, as explained elsewhere62, have also a significant impact, with quantifying effects over GQDs emission or chromophores emission in general. GQDPEIs sensitive response to the light trapping present in NWs arrays could be extended to other studies relevant for optical phenomena exhibited by various geometries and dimensions of nanowires. Thus, EQE maximization at varying potentials (Fig. S7 (a), Supporting Information) in the shortwavelength region is consistent with absorption and PL intensity quenching (as described in Fig. 7 (c) – inset) of GQDPEIs under SiNWs light trapping effect, which strongly implies that energy transfer via GQDPEIs involving both partial photon re-absorption and non-radiative resonance energy transfer have occurred at a high rate. The non-radiative FRET (Föster resonance energy transfer) mechanism enabled by GQDPEIs under the assumption of direct attachment on the surface of SiNWs, is characterized by the efficiency of the transfer rate (EFRET), which is inversely proportional to the third power of the distance 1/r3, where r is the perpendicular distance between the donor and acceptor planes, for the case of metal films acceptors63. Another approach to calculate EFRET involves the measurement of time-resolved PL decay curves of GQDPEIs (Donor) in the presence and absence of SiNWs (Acceptor)64, as shown in Fig. S12 (Supporting Information). A biexponential function was used to fit each of the decay curves, and the corresponding weighted-average decay lifetime was calculated as follows21: 〈τ 〉 =

a 1τ 1 + a 2τ 2 a1 + a 2

(8)

where C7 and C are the fast and slow decay times, respectively, and a [%] represents the percentage contribution of each component. Further, EFRET was calculated using the relation61: E FRET = 1 −

τ DA τD

(9)

The heterojunction possess the fastest recombination rate of 610 ps while GQDPEIs have an average lifetime of 830 ps, consequently EFRET reached ~27%. As for incident photons of energy above the bandgap of GQDPEIs, the electron-hole pairs are generated on the GQDPEIs layer, subsequently separated at the SiNWs interface and extracted at the electrodes, when photons of energy less than the bandgap of GQDPEIs are absorbed they are transferred to the Si and efficiently collected by charge transport channels. Consequently, the GQDPEIs/SiNWs acts as a double-banded heterostructure and converts both visible and ultraviolet lights65, properties that could be exploited also in a solar cell configuration that is able to harvest the full solar spectrum.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

4. Conclusions In summary, we synthetized poly(ethyleneimine) functionalized graphene quantum dots (GQDPEIs) with tuned photoluminescence emission and electronic properties that made them adequate for integrated in silicon based optoelectronic devices. Then, a novel GQDPEIs/p-type SiNWs core-shell heterojunction was fabricated via cost-effective solution processing approach, exhibiting extraordinary photodetection performances. We achieved significant enhancement of the GQDPEIs based heterostructure photoresponse in comparison with the SiNWs reference structures. It is particularly remarkable the major improvement of the EQE values that far exceed 100%. Beside the extension of light absorption due to the large band gap GQDPEI, usually achievable when GQDs are used in hybrid devices, the newly proposed type of QDs, in particular, brings supplementary essential advantages. Thus, experimental analysis indicated that the large enhancement achieved for ITO/nL-GQDPEIs/pSiNWs/Al heterostructures arises from the interplay of various mechanisms, associated with both carriers photogeneration and transport in the heterojunction, mediated by: (i) efficient suppression of the dark current governed by the type I alignment in energy levels; (ii) generation of extra charges in the device determined by the presence of the PEI induced electron trap levels that foster carrier injection on nanowires, and charge photomultiplication effect, showed in the photoresponse results; (iii) photon downconversion effect and efficient energy transfer confirmed by PL quenching. Undoubtedly, the in-depth understanding of GQDPEIs/SiNWs interface phenomena may also impact the optimization of future high-efficient silicon-organic heterojunctions devices.

ASSOCIATED CONTENT Supporting Information Additional results are included: Electrochemical characterization of GQDPEI – finding HOMO-LUMO energy positions; XRD analyses; Optimization steps in photodetector fabrication; Schottky heterojunction parameters; Photo to dark current ratio as function of incident power and key metrics at 1V and 10 V biases; Figure of merit for ITO/6L-GQDs/p-SiNWs/Al heterostructure and evaluation in correlation with similar structures; Capacitance–voltage analyses; Photoluminescence analyses.

ACKNOWLEDGEMENT This work was supported by project no. PN-II-RU-TE-2014-4-1095 of the Romanian National Authority for Scientific Research and Innovation, CNCS – UEFISCDI. 18 ACS Paragon Plus Environment

Page 19 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES 1

Ghosh, R.; Gir, P. K. Silicon Nanowire Heterostructures for Advanced Energy and Environmental

Applications: A Review. Nanotechnology 2017, 28, 012001. 2

Zhang, A.; Kim, H. K.; Cheng, J.; Lo, Y. H. Ultrahigh Responsivity Visible and Infrared Detection Using

Silicon Nanowire Phototransistors. Nano Lett. 2010, 10, 2117–2120. 3

Stelzner, T.; Pietsch, M.; Andrä, G.; Falk, F.; Ose, E.; Christiansen, S. Silicon Nanowire-Based Solar Cells.

Nanotechnology 2008, 19, 295203. 4

Cui, Y.; Zhong, Z. H.; Wang, D. L.; Wang, W. U.; Lieber, C. M. High Performance Silicon Nanowire Field

Effect Transistor. Nano Lett. 2003, 3, 149–152. 5

Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P.

Enhanced Thermoelectric Performance of Rough Silicon Nanowires. Nature 2008, 451, 163–165. 6

Thissandier, F.; Gentile, P.; Pauc, N.; Brousse, T.; Bidan, G.; Sadkia, S. Tuning Silicon Nanowires Doping

Level and Morphology for Highly Efficient Micro-Supercapacitors. Nano Energy 2014, 5, 20-27. 7

Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Nanowire Nanosensors for Highly Sensitive and Selective

Detection of Biological and Chemical Species. Science 2001, 293, 1289–1292. 8

Huang, Z.; Geyer, N.; Werner, P.; de Boor, J.; Gösele, U. Metal-Assisted Chemical Etching of Silicon: A

Review. Adv. Mater. 2011, 23, 285–308. 9

Logeeswaran, V. J.; Jinyong, O.; Avinash, P. N.; Katzenmeyer, A. M.; Gilchrist, K. H.; Grego, S.; Kobayashi,

N. P.; Wang, S.-Y.; Talin, A. A.; Dhar, N. K.; Islam, M. S. A Perspective on Nanowire Photodetectors: Current Status, Future Challenges, and Opportunities. IEEE J. Sel. Top. Quantum Electron. 2011, 17, 1002-1032. 10

Jiang, Y.; Li, C.; Cao, W.; Jiang, Y.; Shang, S.; Xia, C. Large Scale Fabrication of Well-Aligned Cds/P-Si

Shell/Core Nanowire Arrays for Photodetectors Using Solution Methods. Phys. Chem. Chem. Phys. 2015, 17, 16784-16790.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11

Page 20 of 32

Manna, S.; Das, S.; Mondal, S. P.; Singha, R.; Ray, S. K. High Efficiency Si/CdS Radial Nanowire

Heterojunction Photodetectors Using Etched Si Nanowire Templates. J. Phys. Chem. C 2012, 116, 7126−7133. 12

Akgul, F. A.; Akgul, G.; Gullu, H. H.; Unalan, H. E.; Turan, R. Enhanced Diode Performance in Cadmium

Telluride–Silicon Nanowire Heterostructures, J. Alloys Compd. 2015, 644, 131-139. 13

Akgul, F. A.; Akgul, G.; Gullu, H. H. Unalan, H. E.; Turan R. Improved Diode Properties in Zinc Telluride

Thin Film-Silicon Nanowire Heterojunctions. Philos. Mag. 2015, 95, 1164–1183. 14

Kang, H.; Park, J.; Choi, T.; Jung, H.; Lee, K. H.; Im, S.; Kim, H. n-ZnO:N/p-Si Nanowire Photodiode

Prepared by Atomic Layer Deposition. Appl. Phys. Lett. 2012, 100, 041117. 15

Rasool, K.; Rafiq, M. A.; Ahmad, M.; Imran, Z.; Hasan, M. M. TiO2 Nanoparticles and Silicon Nanowires

Hybrid Device: Role of Interface on Electrical, Dielectric, and Photodetection Properties. Appl. Phys. Lett. 2012, 101, 253104. 16

Rasool, K.; Rafiq, M. A.; Ahmad M.; Imran, Z.; Batool, S. S.; Hasan, M. M. Photodetection and Transport

Properties of Surface Capped Silicon Nanowires Arrays with Polyacrylic Acid. AIP Adv. 2013, 3, 082111. 17

Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Liber, C. M. Coaxial Silicon

Nanowires as Solar Cells and Nanoelectronic Power Sources. Nature 2007, 449, 885-889. 18

Luo, L.-B.; Zeng, L.-H.; Xie, C.; Yu, Y.-Q.; Liang, F.-X.; Wu, C.-Y.; Wang, L.; Hu, J.-G.; Light Trapping

and Surface Plasmon Enhanced High-Performance NIR Photodetector. Sci. Rep. 2014, 4, 3914. 19

Li, X.; Rui, M.; Song, J.; Shen, Z.; Zeng, H. Carbon and Graphene Quantum Dots for Optoelectronic and

Energy Devices: A Review. Adv. Funct. Mater. 2015, 25, 4929-4947. 20

Paulo, S.; Palomares, E.; Martinez-Ferrero, E. Graphene and Carbon Quantum Dot-Based Materials in

Photovoltaic Devices: From Synthesis to Applications. Nanomater. 2016, 6, 157. 21

Mihalache, I.; Radoi, A.; Mihaila, M.; Munteanu, C.; Marin, A.; Danila, M.; Kusko, M.; Kusko, C. Charge

and Energy Transfer Interplay in Hybrid Sensitized Solar Cells Mediated by Graphene Quantum Dots. Electrochim. Acta. 2015, 153, 306-315. 22

Zhang, S.; Li, Y.; Song, H.; Chen, X.; Zhou, J.; Hong, S.; Huang, M. Graphene Quantum Dots as The

Electrolyte for Solid State Supercapacitors. Sci. Rep. 2016, 6, 19292. 23

Mihalache, I.; Radoi, A.; Munteanu, C.; Kusko, M.; Kusko, C. Charge Storage and Memory Effect in

Graphene Quantum Dots–PEG600 Hybrid Nanocomposite. Org. Electron. 2014, 15, 216-225. 24

Lu, Q.; Zhang, Y.; Liu, S. Graphene Quantum Dots Enhanced Photocatalytic Activity of Zinc Porphyrin

toward the Degradation of Methylene Blue under Visible-Light Irradiation. J. Mater. Chem. A 2015, 3, 85528558.


Page 21 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

25


Song, S. H.; Jang, M. H.; Chung, J.; Jin, S. H.; Kim, B.H.; Hur, S. H.; Yoo, S.; Cho, Y. H.; Jeon, S. Highly

Efficient Light-Emitting Diode of Graphene Quantum Dots Fabricated from Graphite Intercalation Compounds. Adv. Opt. Mater. 2014, 2, 1016-1023. 26

Zhang, Q.; Jie, J.; Diao, S.; Shao, Z.; Zhang, Q.; Wang, L.; Deng, W.; Hu, W.; Xia, H.; Yuan, X.; Lee, S.-T.

Solution-Processed Graphene Quantum Dot Deep-UV Photodetectors, ACS Nano 2015 9, 1561-1570. 27

Ghosh, D.; Kapri, S.; Bhattacharyya, S. Phenomenal Ultraviolet Photoresponsivity and Detectivity of

Graphene Dots Immobilized on Zinc Oxide Nanorods. ACS Appl. Mater. Interfaces 2016, 8, 35496–35504. 28

Lee, S.-W.; Choi, K.-J.; Kang, B.-H.; Lee, J.-S.; Kim, S.-W.; Kwon, J.-B.; Gopalan, S.-A.; Bae, J.-H.; Kim,

E.-S.; Kwon, D.-H.; Kang, S.-W. Low Dark Current and Improved Detectivity of Hybrid Ultraviolet Photodetector Based on Carbon-Quantum-Dots/Zinc-Oxide-Nanorod Composites. Organic Electronics 2017, 39, 250-257. 29

Kim, C. O.; Hwang, S. W.; Kim, S.; Shin, D. H.; Kang, S. S.; Kim, J. M.; Jang, C. W.; Kim, J. H.; Lee, K.

W.; Choi, S. H.; Hwang, E. High-Performance Graphene-Quantum-Dot Photodetectors. Sci. Rep. 2014, 4, 5603. 30

Li, M.; Zhao, R.; Su, Y.; Yang, Z.; Zhang, Y. Carbon Quantum Dots Decorated Cu2S Nanowire Arrays for

Enhanced Photoelectrochemical Performance. Nanoscale 8 2016 8559-8567. 31

Tian, J.; Zhao, H.; Quan, X.; Zhang, Y.; Yu, H.; Chen, S. Fabrication of Graphene Quantum Dots/Silicon

Nanowires Nanohybrids for Photoelectrochemical Detection of Microcystin-LR. Sens. Actuators, B 2014, 196, 532-538. 32

Pan, J.; Sheng, Y.; Zhang, J.; Huang, P.; Zhang, X.; Feng, B. Photovoltaic Conversion Enhancement of a

Carbon Quantum Dots/p-type CuAlO2/n-type ZnO Photoelectric Device. ACS Appl. Mater. Interfaces 2015, 7, 7878-7883. 33

Xie, C.; Nie, B.; Zeng, L.; Liang, F.-X.; Wang, M.-Z.; Luo, L.; Feng, M.; Yu, Y.; Wu, C.-Y.; Wu, Y.; Yu, S.-

H. Core–Shell Heterojunction of Silicon Nanowire Arrays and Carbon Quantum Dots for Photovoltaic Devices and Self-Driven Photodetectors. ACS Nano 2014, 8, 4015-4022. 34

Akari, S.; Matthes, T. W.; Sommerhalter, C.; Boneberg, J; Leiderer, P. Qualitative Change of the Electronic

Properties of a Semiconductor Surface by Chemical Treatment using Polyelectrolytes. Langmuir 1997, 13, 4369-4371. 35

Freeman, D. D.; Choi, K.; Yu, C. N-Type Thermoelectric Performance of Functionalized Carbon Nanotube-

Filled Polymer Composites. PLoS ONE 2012, 7, e47822. 36

Dong, Y.; Pang, H.; Yang, H. B.; Guo, C.; Shao, J.; Chi, Y.; Li, C. M.; Yu, T. Carbon-Based Dots Co-Doped

with Nitrogen and Sulfur for High Quantum Yield and Excitation-Independent Emission. Angew. Chem. Int. Ed. 2013, 52, 7800–7804. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

37

Page 22 of 32

Xu, W.; Peng, B.; Chen, J.; Liang, M.; Cai, F. New Triphenylamine-Based Dyes for Dye-Sensitized Solar

Cells. J. Phys. Chem. C 2008, 112, 874-880. 38

Dong, Y.; Ruixue W.; Hao L.; Jingwei S.; Yuwu C.; Xiaomei L.; Guonan C. Polyamine-Functionalized

Carbon Quantum Dots for Chemical Sensing. Carbon 2012, 50, 2810-2815. 39

Tang, L.; Ji, R.; Li, X.; Teng, K. S.; Lau, S. P. Size-Dependent Structural and Optical Characteristics of

Glucose-Derived Graphene Quantum Dots. Part. Part. Syst. Charact. 2013, 30, 523–531. 40

Jung, B. O.; Kim, D. C.; Kong, B. H.; Lee, J. H.; Lee, J. Y.; Cho, H. K. Direct Formation of Transparent ITO

Top Electrodes on High-Density ZnO Nanowires by Magnetron Sputtering. Electrochem. Solid-State Lett. 2011, 14, H446-H449. 41

Schroder, D.K. Semiconductor Material and Device Characterization, 3rd ed., John Wiley & Sons, New

York, NY, 2006. 42

Wu, C.-Y.; Pan, Z.hi-Q.; Wang, Y.-Y.; Ge, C.-W.; Yu, Y.-Q.; Xu, J.-Y.; Wang, L.; Luo, L.-B. Core–Shell

Silicon Nanowire Array–Cu Nanofilm Schottky Junction for a Sensitive Self-Powered Near-Infrared Photodetector. J. Mater. Chem. C 2016, 4, 10804-10811. 43

Masala, S.; Adinolfi, V.; Sun, J.-P.; Gobbo, S. D.; Voznyy, O.; Kramer, I. J.; Hill, I. G.; Sargent, E. H. The

Silicon: Colloidal Quantum Dot Heterojunction. Adv. Mater. 2015 27, 7445–7450. 44

Rawat, G.; Somvanshi, D.; Kumar, Y.; Kumar, H.; Kumar C.; Jit, S. Electrical and Ultraviolet - a Detection

Properties of e-Beam Evaporated N-Tio2 Capped p-Si Nanowires Heterojunction Photodiodes. IEEE Trans. Nanotechnol. 2017, 16, 49-57. 45

Bian, J.; Huang, C.; Wang, L.; Hung, T.; Daoud, W. A.; Zhang, R. Carbon Dot Loading and Tio2 Nanorod

Length Dependence of Photoelectrochemical Properties in Carbon Dot/TiO2 Nanorod Array Nanocomposites. ACS Appl. Mater. Interfaces 2014, 6, 4883–4890. 46

Li, J.; Niu, L.; Zheng, Z.; Yan, F. Photosensitive Graphene Transistors. Adv. Mater. 2014, 26, 5239–5273.

47

Bae, J.; Kim, H.; Zhang, X. M.; Dang, C. H.; Zhang, Y.; Choi, Y.J.; Nurmikko, A.; Wang, Z. L. Si Nanowire

Metal-Insulator-Semiconductor Photodetectors as Efficient Light Harvesters. Nanotechnology 2010, 21, 095502 48

Diao, S.; Zhang, X.; Shao, Z.; Ding, K.; Jie, J.; Zhang, X. 12.35% Efficient Graphene Quantum Dots/Silicon

Heterojunction Solar Cells Using Graphene Transparent Electrode. Nano Energy 2017, 31, 359–366. 49

Lu, J.; Liu, H.; Deng, S.; Zheng, M.; Wang, Y.; van Kan, J. A.; Tang, S. H.; Zhang, X.; Sow, C. H.;

Mhaisalkar, S. G. Highly Sensitive and Multispectral Responsive Phototransistor Using Tungsten-Doped VO2 Nanowires. Nanoscale 2014, 6, 7619-7627. 50

Chowdhury, P.; Fortin, P.; Suppes, G.; Holdcroft, S. Aqueous Photoelectrochemical Reduction of

Anthraquinone Disulfonate at Organic Polymer Films. Macromol. Chem. Phys. 2016, 217, 1119−1127. 22 ACS Paragon Plus Environment

Page 23 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

51


Lao, C. S.; Park, M.-C.; Kuang, Q.; Deng, Y.; Sood, A. K.; Polla, D. L.; Wang, Z. L. Giant Enhancement in

UV Response Of ZnO Nanobelts by Polymer Surface-Functionalization. J. Am. Chem. Soc. 2007, 129, 1209612097. 52 53

Anderson, R. L. Experiments on Ge-GaAs Heterojunctions. Solid-State Electron. 1962, 5, 341-344. Gu, Y.-Z.; Lu, H.-L.; Zhang, Y.; Wang, P.-F.; Ding, S.-J.; Zhang, D. W. Effects of ZnO Seed Layer

Annealing Temperature on the Properties of n-ZnO NWs/Al2O3/p-Si Heterojunction. Opt. Express 2015, 23, 24456-24463. 54

Chen, H.-Y.; Lo, M. K. F.; Yang, G.; Monbouquette, H.G.; Yang, Y. Nanoparticle-Assisted High

Photoconductive Gain in Composites of Polymer and Fullerene. Nat. Nanotechnol. 2008, 3, 543-547. 55

Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P. R.; Park, J.; Bao, X. Y.; Lo, Y. H.; Wang, D. ZnO

Nanowire UV Photodetectors with High Internal Gain. Nano Lett. 2007, 7, 1003–1009. 56

Shen, L.; Fang, Y.; Dong, Q.; Xiao, Z.; Huang J. Improving the Sensitivity of a Near-Infrared Nanocomposite

Photodetector by Enhancing Trap Induced Hole Injection. Appl. Phys. Lett. 2015, 106, 023301. 57

Badrana, R. I.; Umar, A.; Al-Heniti, S.; Al-Hajry, A.; Al-Harbi, T. Synthesis and Characterization of Zinc

Oxide Nanorods on Silicon for the Fabrication of p-Si/N-ZnO Heterojunction Diode, J. Alloys Compd. 2010, 508, 375-379. 58

Chen, C.; Qiao, H.; Lin, S.; Luk, C. M.; Liu, Y.; Xu, Z.; Song, J.; Xue, Y.; Li, D.; Yuan, J.; Yu, W.; Pan, C.;

Lau, S. P.; Bao, Q. Highly Responsive MoS2 Photodetectors Enhanced by Graphene Quantum Dots. Sci. Rep. 2015, 5, 11830. 59

Yu, X.; Shen, X.; Mu, X.; Zhang, J.; Sun, B.; Zeng, L.; Yang, L.; Wu, Y.; He, H.; Yang, D. High Efficiency

Organic/Silicon-Nanowire Hybrid Solar Cells: Significance of Strong Inversion Layer. Sci. Rep. 2015, 5, 17371. 60

Zhang, Y.; Xu, J.; Shi, S.; Gao, Y.; Wang, C.; Zhang, X.; Yin, S.; Li L. Development of Solution-Processed

Zno Nanorod Arrays based Photodetectors and the Improvement of UV Photoresponse via AZO Seed Layers. ACS Appl. Mater. Interfaces 2016, 8, 22647-22657. 61

Tataroğlu, A.; Altındal, Ş. Characterization of Current–Voltage (I–V) and Capacitance–Voltage–Frequency

(C–V–f) Features of Al/SiO2/p-Si (MIS) Schottky Diodes. Microelectron. Eng. 2006, 83, 582–588. 62

Fountaine, K. T.; Whitney, W. S.; Atwater, H. A. Resonant Absorption in Semiconductor Nanowires and

Nanowire Arrays: Relating Leaky Waveguide Modes to Bloch Photonic Crystal Modes. J. Appl. Phys. 2014, 116, 153106-1 - 153106-6. 63

Kuhn, H. Classical Aspects of Energy Transfer In Molecular Systems. J. Chem. Phys. 1970, 53, 101-108.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

64

Page 24 of 32

Subramanian, A.; Zhenghui, P.; Genlan, R.; Hongfei, L.; Lisha, Z.; Wanfei, L.; Yongcai Qiu et Xu, Y.; Hou,

Y.; Zheng, Z.; Zhang, Y. Graphene Quantum Dot Antennas for High Efficiency Förster Resonance Energy Transfer based Dye-Sensitized Solar Cells. J. Power Sources 2017, 343, 39-46. 65

Kathalingam, A.; Kim, H.-S.; Park, H.-M.; Valanarasu, S.; Mahalingam, T. Effect of Indium on Photovoltaic

Property of n-Zno/p-Si Heterojunction Device Prepared using Solution-Synthesized ZnO Nanowire Film. J. Photonics Energy 2015, 5, 053085-1 - 053085-15.


Page 25 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

(c)

(d)

(e) Figure 1: (a) TEM image of GQDPEIs with size distribution (b); (c) UV-Vis absorbance spectrum of GQDPEIs in aqueous solution and cast as a film (dotted red line); Photoluminescence emission spectra of GQDPEIs at different excitation wavelengths; inset: normalized spectra (d); Photoluminescence decay profiles of GQDPEIs measured at room temperature in aqueous solution (e). For the lifetime measurements samples were excited with 375 nm picosecond pulsed laser (EPL-375, 375nm, 5mW) and the emission was set at 573 nm.

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 26 of 32

(b)

(c)

(d)

Figure 2. (a) Tilt-view SEM image of ITO/6L-GQDPEIs/SiNWs/Al arrays. Left and right insets show a crosssectional view of p-SiNWs arrays and the plan view of 6L-GQDPEIs/p-SiNWs arrays, respectively; (b) SEM image corresponding to high magnification of (a) and the inset displays the shape of core-shell nanowires heterostructures; (c) TEM image of heterostructure cross-section showing ITO and 6L-GQDPEIs on Si nanowire (left side) and a bundle of core-shell nanowires (inset graph); right side show the high-resolution TEM image of the selected area displaying closely-packed GQDPEIs attached on the nanowire; (d) EDX profile taken from ITO/6L-GQDPEIs/SiNWs - top/middle/bottom zones. ACS Paragon Plus Environment

Page 27 of 32

DARK ILLUM

(a)

REF REF

3L 3L

6L 6L

9L 9L

12L 12L

(b)

REF

3L

6L

9L

12L

100

Ilight / Idark

0.01

Current (A)

1E-3 1E-4

10

1E-5 1E-6 1E-7 -4

(c)

-2

0

REF

-2

10

5

10

2 4 Voltage (V)

3L 15

20

6

30

35

1

10

0

2

4

6

8

10

Voltage (V)

6L 25

8

9L 40

45

(d)

12L 50 mW/cm

2

REF

100

3L

6L

9L

12L

Iph/Idark

80 Current (A)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3

10

60 40 20 2

-4

10

Vbias = 4V

Vbias = 4V 1

-5

10

50

100 150 Time (s)

200

250

0

10

20 30 40 Power (mW/cm2)

50

Figure 3. (a) Current-voltage characteristics of the fabricated heterostructures, under dark and illumination condictions; (b) Light to dark current ratio as functions of bias voltage; (c) Dynamic photocurrent response at different illumination power (4V bias); (d) Photo to dark current ratio as function of incident power.



Vbias = 4V

20

(b)

10

x 10

REF 3L 6L 9L 12L

8

6

400

500

600

700

4

x 10

2

0 400

0 800

500

3000

2000

x 10 1000

0 400

500

600

700

(d)

700

800

800

E-3L E-6L E-9L E-12L

40 EQE Enhancement @ 4V

Vbias = 4V

REF 3L 6L 9L 12L

4000

600

Wavelength (nm)

Wavelength (nm)

(c)

Vbias = 4V

10

1/2

REF 3L 6L 9L 12L

12

Responsivity (A / W)

30

Detectivity (10 cm Hz / W)

(a)

EQE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

30

20

10

0

Wavelength (nm)

400

500

600 700 Wavelength (nm)

800

Figure 4. Spectral responsivities (a) and detectivities (b) of the test heterostructures at 4V bias. Spectral quantum efficiencies (c) and the EQE enhancements (d) of the test heterostructures at 4V bias.


Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

(a)

(d)

(c)

Figure 5. Energy band diagrams of the fabricated photodetectors: ITO/p-SiNWs/Al reference structure in dark (a) and light illumination (b) conditions; and ITO/nL-GQDPEIs/p-SiNWs/Al heterojunction under reverse bias in dark (c) and light illumination (d) conditions, respectively. ACS Paragon Plus Environment


6L

30

20

9L

12L

(b)

0,0

3

3

2

-0,1 2

1

0 -0,8

-0,6

-0,4

10

-0,2

0,0

0,2

0,4

-0,2

1

Voltage (V)

0 -0,8

-0,3

-0,6

-0,4

-0,2

0,0

0,2

0 0L

0,4

3L

REF

REF

1020

3L

6L

9L

(d)

12L

6L

9L

12L

Layer number

Voltage (V)

(c)

17 -3 N (10 cm )

3L 1/C2 x 1017 (F-2)

Capacitance (nF)

REF

Vbi (V)

(a)

REF

3L

6L

9L

12L

15

Capacitance (nF)

N (cm-3)

1019

1018

1017

1016

10

5

1015

0 1014 0,0

0,2

0,4

0,6

0,8

1,0

1

(e)

REF 0.1

3L 0.2

0.4

6L 0.6

10

100

1000

Frequency (kHz)

Wd (a.u.)

9L 0.8

12L 1 SUN

20

Capacitance (nF)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

15

10

5

0 0

100

200

300

400

500

600

Time (s)

Figure 6. (a) Variation of the capacitance with the applied voltage and plots corresponding to Mott-Schottky analysis (inset) at 5kHz; (b) Carrier concentration and built-in potential obtained from capacitance–voltage (C–V) data; (c) Carrier concentration vs depth profiles; (d) Frequency dependence of the electrical capacitance; (e) Transient photocapacitance analysis at 0.1 V bias, 5 kHz frequency.


Page 31 of 32

Normalized Absorbance

(a) GQDPEI emission

3000

1.2

2000

0.8

1000

0.4

GQDPEI 200

PL Intensity (a.u.)

SiNWs

300

400 500 600 Wavelength (nm)

700

800

@ 580nm Peak Position

PL Intensity (a.u.)

PLE Intensity (a.u.)

(b)

4000

2000

GQD

PEI s/SiNWs

1000 0 360

400

440

480

520

Wavelength (nm)

500

550 600 650 700 Wavelength (nm)

GQDPEI/SiNWs

(c)

520 500 480 460 440 420 400 380 360 340 320

PEI GQD s film

3000

320

450

750

800

6.0

% A

1,8

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3L 6L 9L 12L

1,6

4.5 3.0 1.5 400

500

600

700

Wavelength (nm)

SiNWs 1,4

1,2 400

500

600

700

800

Wavelength (nm)

Figure 7. (a) Schematic of the energy transfer process based on absorbance spectrum of SiNWs array and absorbance and PL spectrum of GQDPEI, respectively (the patterned area indicates the spectral overlap) (b) PL spectra recorder for 6L-GQDPEI/SiNWs sample. Inset: Evolution of the peak intensity in GQDPEIs /SiNWs heterojunction compared to GQDPEIs film. (c) Absorbance spectra before and after loading nL-GQDs layers. Inset: The percentage change in the absorbance with respect to SiNWs values, when different layers of GQDs are added;



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Graphical abstract:


Page 32 of 32

Engineering graphene quantum dots for enhanced ultraviolet and

Recommend Documents