Shifting the Photoresponse of ZnO Nanowires into the Visible Spectral

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Shifting the Photoresponse of ZnO Nanowires into the Visible Spectral Range by Surface Functionalization with Tailor-Made Carbon Nanodots Kseniia Zimmermann, Frank Dissinger, Davide Cammi, Angelina Jaros, Florian Meierhofer, Siegfried R Waldvogel, and Tobias Voss J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09308 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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The Journal of Physical Chemistry

Shifting the Photoresponse of ZnO Nanowires into the Visible Spectral Range by Surface Functionalization with Tailor-Made Carbon Nanodots Kseniia Zimmermann†, Frank Dissinger§, Davide Cammi†, Angelina Jaros†, Florian Meierhofer†, Siegfried R. Waldvogel§, and Tobias Voss†* †Institute of Semiconductor Technology and Laboratory for Emerging Nanometrology (LENA), TU Braunschweig, Braunschweig, Germany §Institute of Organic Chemistry, Johannes Gutenberg University, Mainz, Germany

ABSTRACT. We report on the surface functionalization of ZnO Nanowires (NWs) with specifically synthesized Carbon Nanodots (C-Dots, CDs), that allow us to shift the photoresponse of the NWs far into the visible spectral range. We modified a well-established citric acid based synthesis protocol for C-Dots by substituting the commonly used aliphatic amine precursors with 2,3-diaminopyridine (CDs-1) and 2,3-diaminonaphthalene (CDs-2), respectively. After surface functionalization, we achieve more than a 100-fold increase of the photoresponse of ZnO NW

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photodetectors at 2.92 eV (425 nm) with the CDs-1 and a more than a 20-fold increase at 2.75 eV (450 nm) with the CDs-2. The enhanced absorption of the C-Dots in the visible spectral range is attributed to the formation of additional chemical bonds between the carboxyl moieties of the citric acid and the amine groups of the 2,3-diaminopyridine and 2,3-diaminonaphthalene which is underlined by results from FTIR spectroscopy. We present a model for the microscopic origin of the enhanced photoconductivity which is based on shifts of the LUMO energy levels in the C-Dots synthesized with different amine precursors relative to the conduction band minimum of the ZnO nanowires.

1.INTRODUCTION The design of hybrid nanostructured devices based on various material components is a rapidly growing field of material research since it allows for novel applications as well as expandability of the application area of already existing materials. With the successful combination of different materials with complementary properties, a tremendous enhancement of the resulting application’s performance can be obtained. Among semiconductor nanomaterials, ZnO nanowires (NWs) are excellent candidates for photonic and optoelectronic applications1, based on remarkable properties such as their wide band gap (≈ 3,37 eV at room temperature2), non-toxicity,3 flexibility in fabrication methods4, the possibility of various chemical modification methods due to the termination of different crystal facets5,6 and large surface-to-volume ratio.7 Because of their large room-temperature bandgap, the application of ZnO NWs in opto-electronic devices is mostly limited to the blue-UV spectral range. Surface functionalization of ZnO NWs with colloidal semiconductor quantum dots or organic dyes


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has recently emerged as a tool for enhancing their photoresponse towards the visible spectral range.8,9,10,11 However, most of the investigated quantum dots based on cadmium and lead chalcogenides are severely toxic, impeding the application of such devices in larger scales.12,13,14 In addition, most of the organic dyes quickly degrade due to the photocatalytic properties of ZnO nanostructures.15,16 Carbon nanodots (C-Dots) have recently emerged as an alternative to traditional semiconductor quantum dots and organic dyes. They draw substantial attention due to their low cytotoxicity,17 inexpensive and yet easy to accomplish, large-scale manufacturing processes and the possibility to control their optical properties via variation of chemical precursors.18,19 Therefore, it is possible to prepare C-Dots with absorbance in a wide spectral range.20 By fabrication of a nanostructured hybrid device consisting of ZnO NWs decorated with C-Dots, obtained from a widely used synthesis protocol, we recently demonstrated the enhancement of the resulting photoconductivity below the ZnO band gap and discussed the charge-carrier dynamics.21 In this work, we have tailored the absorption of the C-Dots via modification of their synthesis protocol22 and incorporation of the aromatic amines 2,3- diaminopyridine (CDs-1) and 2,3 diaminonaphthalene (CDs-2) instead of the formerly used ethylenediamine end-capped polyethyleneimine (PEI-EC) (CDs-Ref). We performed absorption measurements in UV-VIS and IR spectral regions of the as-prepared C-Dots, as well as photoluminescence (PL) measurements of bare and functionalized ZnO NWs. Furthermore, we investigated the photoconductivity of NW arrays, functionalized with the tuned C-Dots under optical excitation with photon energies below the band gap in the spectral range from 3.26 eV to 2.10 eV. Compared to the pristine ZnO NWs, functionalized devices show an enhanced photoresponse with maxima in the blue spectral region. The positions of the photoresponse maxima can be shifted towards lower excitation energies


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relative to the band-edge absorbance of bare ZnO NWs. We also observed a substantial enhancement of the photoconductivity of the ZnO NWs at even 2.75 eV (450 nm). The redshift of the photoresponse depends on the absorption maximum of the used C-Dot species, which is dependent on the used amine precursor and a size of the aromatic domains in the C-Dots. Furthermore, we propose an energy-level model to explain the observed results.

2.EXPERIMENTAL 2.1 Synthesis of C-Dots The C-Dots (CDs-Ref, CDs-1 and CDs-2) were synthesized by low-temperature solvothermal decomposition of citric acid monohydrate (CA, Merck) and selected amine precursor, respectively. Three different amines: ethylenediamine end-capped polyethyleneimine (PEI-EC, Mw ~ 800, Sigma-Aldrich), 2,3-diaminopyridine (2,3-DAP) (Sigma-Aldrich) and 2,3-diaminonaphthalene (2,3-DAN) (Sigma-Aldrich) were chosen following modified procedures reported elsewhere.19,22 For the solvothermal pyrolysis, equal molar amounts of citric acid and respective amineprecursor were mixed in 20 mL of de-ionized water (CDs-1, CDs-2). For CDs-Ref 2.0 g CA and 0.5 g PEI-EC were used. Each mixture was placed in an open round bottom flask and heated up to 180 °C for approximately 40 min. The evaporated solvent was replaced in several 5 mL portions by de-ionized water until a yellow-brown gel formed. After the completion of the reaction, the mixture was cooled to ambient temperature, and purified by column chromatography on silica. For CDs-1 and CDs-2 methanol (Acros Organics) was used as eluent, for CDs-Ref 0.01 M aqueous HCl (Backer) was applied, due to solubility issues and afterwards neutralized with solid


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NaHCO3. Each sample preparation was finalized by freeze-drying. The chemical structures of citric acid and amine precursors used for all C-Dots syntheses are shown in figure 1 (a).

Figure 1. (a) Chemical structures of used precursors for the synthesis of different types of C-Dots. (b) Schematic drawing of the ZnO nanowire sensor device functionalized with C-Dots. (c) SEM


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image of pristine ZnO nanowires (inset: SEM image with higher magnification), (d) SEM image of ZnO nanowires after functionalization with C-Dots (solvent: ethanol) (inset: SEM image with higher magnification), and (e) I-V characteristics of NW devices before and after functionalization.

2.2 Preparation of ZnO Nanowires Interdigitated finger contacts were fabricated on a sapphire substrate (Crystec) by photolithography and evaporation of 30 nm Ti and 300 nm Au. Afterwards, a positive photoresist was deposited to allow for a precise NW growth on the sensor field and in order to prevent growth on the squared contact pads. The ZnO NWs were grown by a hydrothermal approach on top of the interdigitated Ti/Au contacts. Details of the ZnO growth are described elsewhere.21,23 Figure 1 shows a schematic drawing of the interdigitated contacts on top of the sapphire substrate (b) and an electron-microscopy image (FE-SEM, Zeiss Supra 35) of the ZnO NWs grown on top of the interdigitated contacts (c). The NWs are quasi vertically aligned and have a well-defined hexagonal structure. Their diameter varies from 170 nm to 50 nm, with an average diameter of 90 nm with a density of ~ 109 NWs cm-2.

2.3 Fabrication of ZnO NW / C-Dot hybrid devices The C-Dots were dissolved in ethanol (Merck) to obtain a 0.08 wt% concentration. Afterwards, 20 µL of the desired C-Dot solution was dropped onto the NW arrays between the contact pads, which were kept in air at room temperature for several hours until the complete evaporation of the volatiles on the ZnO NWs. The SEM images of functionalized nanowires are shown in figure 1 (d). After the functionalization with C-Dots, the morphology of ZnO NWs, shown at low magnification in figure


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1 (d), is different compared to that of pristine nanowires at the same magnification in the figure 1 (c). However, the high-resolution SEM image shows that no significant etching of the nanowire surfaces is caused by the functionalization process. We expect that the concentration of C-Dots decreases from the top to the bottom of the nanowires.

2.4 Characterization 2.4.1 Optical Characterization. The absorption measurements of the C-Dot solutions were carried out in 10 mm quartz cuvettes under ambient conditions (Jasco UV-VIS-NIR V670 spectrophotometer). The photoluminescence characterization of the C-Dot solutions, as well as of both pristine and functionalized devices, were performed under excitation with a He-Cd laser (325 nm (3.81 eV), Kimmon) with an Acton SP2300 spectrometer (Princeton Instruments) and an attached liquidnitrogen cooled CCD camera, with maximum laser powers of 6.7 mW and ~1.1 mW (for the liquid and solid samples, respectively) and a spot diameter of ~5 µm (for solid samples). The PL measurements of ZnO NWs were performed with the samples kept in vacuum (5·10-7 mbar) at room temperature in order to reduce oxygen-induced photo-oxidation of the C-Dots. Fourier-transform infrared (FTIR) spectra of C-Dots in powder form were recorded on the attenuated total reflection (ATR) diamond crystal (GladiATR, Pike) using a Jasco FV 6700 IR Spectrometer. 2.4.2 Electrical Characterization. The electrical characterization of both pristine and functionalized devices was performed by the use of a Keithley 2400 source meter unit, applying a voltage difference between the device’s electrodes and measuring the corresponding electric current.


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The photoconductivity measurements were performed by illuminating the samples with a xenon lamp equipped with a 200 mm monochromator, which was used to generate wavelength-tunable excitation light with a bandwidth of 10 nm at each center wavelength between 600 nm and 350 nm. The excitation intensity was around 40, 80 and 40 μW/cm2 in the blue, green and red spectral regions, respectively.

3.RESULTS AND DISCUSSIONS 3.1 Optical Characterization of C-Dots Figure 2 (a-c) shows the normalized absorbance of the C-Dots dissolved in water (CDs-Ref) and ethanol (CDs-1 and CDs-2) compared to the normalized absorbance of the corresponding amine precursors dissolved in the same solvents, respectively, for the spectral range between 250 nm (5.0 eV) and 600 nm (2.10 eV). In the spectrum of the CDs-Ref (figure 2 (a), grey line) a new absorption peak appears at 364 nm (3.41 eV), which is not present in the absorption spectrum of PEI-EC at this photon energy (figure 2 (a), black dashed line). The normalized absorbance of CDs-1 (figure 2 (b), blue line) and CDs-2 (figure 2 (c), red line) partly resembles the absorbance of corresponding aromatic amines (figures 2 (b) and (c) in black dashed lines) in the spectral range from 250 nm (4.96 eV) to 354 nm (3.50 eV). Similarly, to the CDs-Ref, new absorption peaks are observed in the spectra of CDs-1 at 398 nm (3.12 eV) and 416 nm (2.98 eV), and CDs-2 at 413 nm (3.0 eV) and 435 nm (2.85 eV), respectively, which are not present in absorption spectra of corresponding amine precursors. In the literature, these absorption peaks are typically assigned to n→π* transitions involving the –NH2 and –C=O groups at the surface of the C-Dots.24,25,26


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Figure 2. Normalized absorbance of C-Dots and their corresponding amine precursors measured in suitable solvents (water or ethanol) in the optical range from 250 to 600 nm (a-c). (a) CDs-Ref in grey and PEI-EC in black, (b) CDs-1 in blue and 2,3-DAP in black, (c) CDs-2 in red and 2,3DAN in black. The CDs-Ref and PEI-EC are measured in water, while CDs-1 and CDs-2 with corresponding amines are measured in ethanol due to different solubility (d) Magnified plot of normalized absorbance (logarithmic scale) of synthesized C-Dot solutions (CDs-Ref in grey, CDs-


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1 in blue, CDs-2 in red) demonstrates a redshift of the absorption maxima in the spectral energy range from 3.5 to 2.1 eV. Figure 2 (d) shows normalized absorption spectra of CDs-Ref (grey line), CDs-1 (blue line) and CDs-2 (red line) (shown in panels (a)-(c)), plotted on a logarithmic scale as a function of the photon energy from 4.25 eV to 2.0 eV, to intensify the changes in the absorption behaviour of these systems at the low energy region. The positions of the n→π* peak maxima of the CDs-1 and CDs-2 are red-shifted relative to that of the CDs-Ref. The maximum redshift of 0.6 eV is observed between the CDs-Ref and CDs-2 (marked with the black bar in figure 2 (d)). This redshift might be attributed to the larger size of the C-Dot core of CDs-2 consisting of sp2 hybridized domains with π-π electron conjugation, and in part also to the incorporation of nitrogen hetero-atoms that will be discussed below.26 To investigate chemical groups present on the surface of the C-Dots, FTIR spectra of the used amine precursors and the resulting C-Dots were measured (figure 3). Figure 3 shows the transmittance of (a) CDs-Ref (grey line), PEI-EC (black line) and citric acid monohydrate (green line), (b) CDs-1 (blue line) and 2,3-DAP (black line), (c) CDs-2 (red line) and 2,3-DAN (black line) in the 1000-4000 cm-1 frequency range.


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Figure 3. ATR-FTIR transmittance spectra of synthesized C-Dots and corresponding amine precursor molecules: (a) CDs-Ref (b) CDs-1, (c) CDs-2. (d) vibrational modes of –C=O group (marked with *) in the range 1500-2000 cm-1 in different C-Dots: CDs-Ref (grey line), CDs-1 (blue line), CDs-2 (red line) relative to the position of the –C=O vibrations in citric acid (green line).


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The summarized vibrational modes corresponding to the various groups present in C-Dots are listed in the table 1. Table 1. IR frequencies and corresponding chemical groups in studied C-Dots and citric acid. The assignments of the corresponding groups are made according to the references.27,28 Observed at wavenumber (cm-1)

Chemical group

CDsRef

CDs-1

CDs-2

CA

Combination of C-H(def) and C-OH(str)

1069

1075

10121100

C-H (def)

1170

1180

1390

-C-N-(vib)

n.a

n.a

1270

n.a

-C=N-(str)

n.a

1570

1585

n.a

Combination of –N-H(def) and –C=C(str)

n.a

-OH(def)

1395

1385

1694

1660 (amide),

-C=O(str)

(amide)

-N-H(str) -C-H(str)

1518

1715

n.a

1403

12881421

1640 (amide)

1721

3310-3430 2954

n.a

n.a n.a

25613023

The IR spectra in figure 3 (a-c) display multiple absorption peaks, which can be attributed to the vibrational modes of specific chemical groups in the investigated C-Dots and amine precursors (Table 1). Two main features are striking:


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1)

New absorption peaks around 1600 cm-1 (marked with ν(C=O) in panels (a-c)) emerge in

the spectra of the C-Dots which are less pronounced in the spectra of the corresponding amine precursors; 2)

Enhanced absorption related to vibrational modes of -N-H- groups in the region of ~ 3000

to 3500 cm-1 (highlighted in blue area in panels (a-c)) have significantly lower intensities in the spectra of C-Dots than of corresponding amine precursors. Figure 3 (d) shows the IR spectra of the studied C-Dots (CDs-Ref in grey, CDs-1 in blue and CDs-2 in red) and the citric acid (green) in the range of 1500-2000 cm-1. This IR region corresponds to the maximum intensity of the vibrations of various carbonyl (-C=O) groups in citric acid and therefore shows a good comparison of the similar vibrations in the C –Dots (labelled with *). The central vibration frequency in citric acid is observed at 1721 cm-1. This vibration is also present in the IR spectra of C-Dots, but the peak position is shifted to lower wavenumbers compared to the position in the spectrum of citric acid. Considering all observation in figures 3 (a-d), we interpret these facts in terms of the formation of a new amide bond (-C=O(-NR’)-) between the carboxyl groups (-C=O(-OH)) on the surface of the C-Dots (as a rest of the citric acid) and the amino-groups of the amine precursors. The shift of the vibration frequency of the –C=O groups in the newly formed amide bond towards lower frequencies compared to that in the citric acid is likely because of the substitution of the –OH groups in the carboxyl group by the heavier amine precursors. Taken together, these results suggest that the –COOH groups on the surface of the C-Dots act as a preferred site for the chemical bonding with the amine precursors.29,30,31 It has been recently shown, that C-Dots synthesized for less than 1 hour mainly contain small fluorophore molecules whose structure is possibly similar to the citrazinic acid, whereas those


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heated for more than 1 hour show the appearance of larger sp2 aromatic domains.29,32 The C-Dots used in this work were prepared for approx. 40 minutes, and this supports the hypothesis that their main structural and optical properties originate from citrazinic acid-based molecules. The formation of these fluorophores may explain the appearance of the new absorption features of the C-Dots in UV-VIS and IR spectral regions (figures 2 and 3). Time-dependent DFT calculations performed on four pyrene-based models with different positioning of nitrogen atoms and one nitrogen-free reference system show that nitrogen atoms incorporated into the carbon lattice are responsible for the absorption redshift due to the narrowing of the HOMO-LUMO gap in C-Dots.33 This statement may be applied to the observed redshift of the n→π* transition in our CDs-1 and CDs-2 which, as already stated above, may originate from the larger size of the sp2 core, and from the incorporation of nitrogen hetero-atoms. The presence of sp2 nitrogen in CDs-1 and CDs-2 was confirmed by our FTIR results (figure 3). Because FTIR is not a surface specific technique, the vibrational modes of –C=N- bonds may belong to the core as well. Considering these facts, we propose a hypothetical structure of our C-Dots (based on the literature reports26,29,32,33,34) that is shown in figure 4.


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Figure 4. Hypothetical structure of C-Dots. The region highlighted in red indicates sp2-carbon structure, also referring as a “core” which is surrounded by attached to it various chemical moieties and referred as a “surface”.

During the first stages of the synthesis, mostly small fluorophore molecules are formed. As the synthesis continues with time, the fluorophores are prone to undergo changes in their inner structure, which results in the formation of the sp2-aromatic domains (highlighted with red circle).32 Another possible path for the formation of sp2-domains is via condensation and dehydratation reactions of the citric acid itself.29 It was shown experimentally, that citrazinic acid derivatives formed during the first stage of the synthesis, can be later attached to the sp2-core via condensation of carboxyl, hydroxyl or amine groups, or may be incorporated into the π-conjugation of the sp2-system. The amine precursor


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molecules can therefore substantially modify the sp2-aromatic interface in order to meet the tailored absorption characteristics for the desired optical application.

3.2 Photoresponse of ZnO NW before and after modification with C-Dots The successful functionalization of the ZnO NWs with CDs-1 and CDs-2 was proved by PL measurements (Supporting Information). I-V measurements were performed to check the electronic properties of the prepared ZnO NW devices before and after functionalization with the compounds CDs-1 and CDs-2. Figure 1 (e) shows the current flowing through the ZnO NW devices before and after functionalization with CDs-1 and CDs-2 in the voltage range of -5 to +5 V measured in the dark. All curves in figure 1 (e) show a linear dependence of the current over the voltage, indicating the formation of Ohmic contacts. The resistance of the unfunctionalized devices is around 17 kΩ, while that of the functionalized devices with CD-1 and CD-2 is around ≈ 30 kΩ and ≈ 60 kΩ, respectively. A similar increase of the resistivity after surface-functionalization was observed for ZnO NWs functionalized with porphyrins.35 Therefore, we attribute the increased resistance in functionalized NWs to the attachment of C-Dots We assume, that C-Dots mainly bind via –OH and –C=O groups to the (10-10) sidewalls of the ZnO NWs.36,37 Due to the high resistance of the ZnO seed layer (Supporting Information), we expect that the charge carriers flow almost exclusively through the multiple junctions formed between the ZnO NWs. To verify the possibility of tailoring the photoresponse via surface functionalization of ZnO NWs with C-Dots, series of photoconductivity measurements were carried out.


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Figure 5. (a) Exemplary current curve (I-Idark) of bare and functionalized ZnO NW devices recorded during 20 s of illumination at 2.75 eV (~ 450 nm) excitation. Note that for clarity reasons the raw data of bare and functionalized with CDs-2 ZnO NWs were multiplied by a factor 100 and 10, respectively. (b) Obtained photocurrent (current amplitude) of studied nanowires treated with CDs-1 and CDs-2 after 20 s illumination over whole excitation range. (c) The reference measurement of the ZnO NWs device treated with ethanol. Figure 5 (a) shows the photocurrent as a function of time for bare ZnO NWs (black line) and NWs functionalized with CDs-1 (blue line), and CDs-2 (red line), measured under illumination with a photon energy of 2.75 eV. The background current (Idark) has been subtracted from all curves. The


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light source was turned on at t=0 s. At t=0 s, an increase in current is observed. The device consisting of bare ZnO NWs shows an increase of the current as long as the illumination is on, while the devices functionalized with CDs-1 and CDs-2 resembles this trend with a ~ 450-fold and ~ 20 -fold current enhancement, respectively. As it is shown in figure 5 (a), the strongest current enhancement after 20 s of light excitation was observed for the device functionalized with CDs-1 (blue line). Before recording a new photoconductivity measurement, the samples were kept in dark until the complete decay of the photocurrent. For a quantitative comparison, the current amplitude (ICA) was calculated as the difference between the background current (Idark) and the current after 20 s of illumination (Imax): (1) ICA = Imax - Idark

Figure 5 (b) shows ICA for bare and functionalized devices plotted as a function of the excitation photon energy on a logarithmic scale. The bare device (shown in black symbols) shows a poor photoresponse (7.4·10-3 – 4.9·10-5 mA) for excitation energies below the band gap energy of ZnO NWs (vertical dotted line). The weak signal obtained in the range between 3.10 eV and 2.25 eV is attributed to the excitation of defect levels in the band gap of ZnO NWs. For energies at and above the ZnO band gap (3.10 - 3.44 eV), ICA increases up to 0.1 mA due to the direct excitation of electron-hole pairs in the ZnO NWs. The devices functionalized with CDs-1 and CDs-2 show an enhanced ICA over the whole investigated spectral range. In particular, the modification with CDs-1 enhances the current amplitude by a factor over 100 compared to the bare ZnO NWs excited at 2.92 eV. In figure 5 (b), the photocurrent of the device functionalized with CDs-1 shows a local maximum at this photon energy. A similar effect is observed for ZnO NWs functionalized with CDs-2 under excitation at photon energy of 2.75 eV, which results in ICA being enhanced by a


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factor more than 20, compared to the bare ZnO NWs. This ICA maximum is redshifted by 0.17 eV compared to the ICA maximum of the device with CDs-1. The observed redshift is qualitatively in good agreement with the redshift of the n→π* transition in the CDs-2 (figure 2 (d)). Thus, these results demonstrate the influence of the absorption properties of the C-Dots on the spectral sensitivity of the hybrid ZnO NW / C-Dot devices. It has been reported that ethanol molecules absorbed on the surface of ZnO NWs can influence their photoconductive properties.38,39 To check the possible influence of ethanol, which was used as solvent for functionalization, we performed additional photoconductivity experiments (figure 5 (c)) on reference devices. Those samples were prepared analogously to the studied samples but were treated with just ethanol without C-Dots for functionalization. Figure 5 (c) shows ICA plotted as a function of the excitation energy on a logarithmic scale for the pristine (black) and ethanol-treated ZnO NW devices (orange), for the energy range from 3.44 eV to 2.10 eV. The NWs treated with ethanol (figure 5 (c)) show a 3-fold ICA decrease in the range between 3.44 eV and 2.75 eV, compared to the bare NWs. The observed results suggest no significant influence of the used solvent on the enhanced photoconductivity, observed after the functionalization of ZnO devices with CDs-1 and CDs-2 suspensions.

3.3 Photoresponse model The increase of the photoconductivity in the functionalized devices (figure 5 (b)) indicates a modification of the electronic properties of the ZnO NWs. The photoresponse enhancement in the functionalized devices can be ascribed to two main factors: (1) a possible injection of the photoexcited electrons from C-Dot’s lowest unoccupied orbital (LUMO) into the conduction band


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(CB) of the ZnO NWs and (2) the increased density of the surface states (DOS) caused by the functionalization with C-Dots. Generally, surface states in semiconductor nanowires play a very important role for the dynamics of photoconductivity, in particular, when the electronic transport is dominated by defect-assisted absorption processes and trapping of charge-carriers by long-lived surface states. The results shown in figure 5 (c) and a previous work21 demonstrate that the surface roughness and the associated defects induced by the etching of ZnO NWs in the quite aggressive chemical environment do not seem to play a major role for the enhanced photoconductivity in the ZnO NWs functionalized with C-Dots. The hydrothermally grown ZnO NWs studied in this work contain rather a substantial density of surface defects associated with oxygen vacancies (despite the postgrowth annealing for ~1 hour at 400°C in air) and additional point defects in the bulk material,40 which is also demonstrated by the photoluminescence spectra shown in figure S1 (Supporting Information). The presence of these defect states in the studied ZnO NWs also explains their photoresponse under illumination with sub-bandgap photon energy (bandgap of the nanowires ~3.25 eV according to the results of PL spectroscopy)41, albeit with a substantially lower sensitivity (Figure 5 (b)). These surface states are very likely to act as adsorption sites for negatively charged molecular oxygen-related species which results in the formation of a non-conducting depletion layer close to the nanowire surface even before the functionalization with C-Dots. Depending on the resulting surface potential and the excess electron density, ZnO NWs may be completely depleted.42,43 However, we observe a relatively long current decay for the functionalized NWs after the illumination is turned off. This current decay is even longer in comparison to that of bare ZnO NWs (Figure 5 (a)). This is due to the fact that under illumination, the weakly bound charged oxygen species interact with the photo-generated holes in ZnO NWs followed by their desorption,


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along with a consequent reduction of the surface band-bending and a reduction of depletion width. If the functionalized ZnO NWs would be completely depleted, the above-mentioned effect could not be observed because of the enhanced surface recombination rate of the photo-generated chargecarriers. In other words, we would observe a fast drop of the current to the dark current values as soon as illumination is switched off. Therefore, the main source of the enhanced optical response below the band gap of ZnO NWs is an electron transfer from the LUMO of the C-Dots to the CB of the ZnO NWs. The possible electron-transfer mechanism between the C-Dots and the ZnO NWs is schematically shown in figure 6 (a).

Figure 6. Proposed physical model of an electron transfer from the C-Dots to the ZnO NWs: (a) Hypothetical alignment of the HOMO-LUMO energy levels in C-Dots relative to the energy positions of VBM and CBM in ZnO NW, (b) Position of the HOMO-LUMO energy levels in CDots relative to the distribution of DOS in ZnO NW. The blue and red colours correspond to the theoretical HOMO-LUMO levels of CDs-1 and CDs-2, respectively. Here: electron transfer (ET),


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valence band (VB), conduction band (CB), valence band maximum (VBM) and conduction band minimum (CBM). The arrangement of the HOMO-LUMO levels of the C-Dots was estimated according to the previous studies.19,44

During the photoexcitation with photon energies below the band gap of ZnO, an electron is excited from the highest occupied molecular orbital (HOMO) to the LUMO in the C -Dots. Our observations suggest that the energy positions of the LUMO in the C-Dots are located energetically higher than the CB of the ZnO NWs, leading to an injection of the excited electron from the LUMO to the CB of the ZnO NWs. We used the previous results19,44 to subtract the relative positions of the HOMO and LUMO levels in C-Dots. Our experimental results show that the exchange of the amine precursors allows narrowing of the HOMO-LUMO gap in the studied C-Dots as compared with the CDs-Ref (figure 2). Thus, it is possible to induce the electron transfer between the C-Dots and the ZnO NWs already at lower photon energies. In case of CDs-2, we have a smaller HOMOLUMO gap than in CDs-1 due to the larger conjugation of amine precursor which results in a redshift of the light absorption, compared to CDs-1 (figure 2). Concerning the absolute values of the photocurrent, we have to consider the relative position of the LUMOs of C-Dots relative to the minimum of the CB of ZnO NWs and the corresponding DOS in the CB of ZnO NWs available for the electron transfer (figure 6 (b)). Figure 6 (b) shows a schematic drawing of the relative positions of the HOMO and LUMO energy levels of the CDots relative to the DOS in the VB and CB of the ZnO NWs. Due to the larger HOMO-LUMO energy gap of the CDs-1 their LUMO will be located at a higher energy level than that of the CDs2, relative to the minimum of the CB of ZnO NWs (figure 6 (a)). It is expected that the DOS for electrons in the CB of the studied ZnO NWs increases with the square root of the electron energy,


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comparable to a bulk system. This is because the average NW diameter is significantly larger than the de Broglie wavelength in ZnO at room temperature (~5 nm45,46) and quantum confinement effects are not expected to play a major role. Thus, a larger DOS in the CB of the ZnO NWs is available for the electron injection from the LUMOCDs-1 than from LUMOCDs-2 (figure 6 (b)). This model explains the larger value of the photocurrent observed in the devices functionalized with CDs-1 over the whole investigated spectral range and gives an opportunity of absorption tailoring towards the visible spectral range for visible-blind ZnO nanostructures.

4.CONCLUSION We investigated the photoconductivity of ZnO NW devices functionalized with different types of carbon nanodots under photoexcitation below the band gap of ZnO. Compared to pristine devices, the devices with CDs-1 (citric acid with 2,3-DAP) and CDs-2 (citric acid with 2,3-DAN) show a local maximum of the photoconductivity under photoexcitation at 2.92 eV and 2.75 eV, with an enhancement of more than a factor 100 and 20, respectively. The photoresponse enhancement in the functionalized devices was interpreted in terms of the electron transfer from the LUMO energy level of C-Dots to the CB of the ZnO NWs. We ascribed the observed photoconductivity shift of ZnO NW devices into the visible spectral range to the redshift of absorption features of CDs-1 and CDs-2. We attributed this absorption redshift to the formation of a new amide bond in combination with the π−π conjugation of the CDots. We confirmed the formation of the new amide bonds between the carboxyl groups in citric acid with amine groups in pyridine and naphthalene derivatives by comparison the FTIR spectra of the C-Dots with these of their precursors.


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The overall results give an opportunity for further development and improvement of environmental benign optoelectronic devices based on C-Dots.

ASSOCIATED CONTENT Supporting Information. The PL spectra of ZnO nanowires before and after the functionalization with C-Dots, are compared to the PL spectra of CDs-1 and CDs-2 dissolved in ethanol, I-V characteristics of the ZnO seed layer prior the nanowire growth and FTIR spectrum of citric acid monohydrate. The following files are available free of charge. brief description (file type, i.e., PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *Tel. +49-531-391-3821 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge the financial support by the DFG Research Training Group GrK1952/1 "Metrology for Complex Nanosystems" and the Braunschweig International


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Graduate School of Metrology B-IGSM and DFG Research Unit FOR 1616 “Dynamics and Interactions of Semiconductor Nanowires for Optoelectronics”. Florian Meierhofer and Tobias Voss acknowledge financial support from “Niedersächsisches Vorab” through “Quantum- and Nano-Metrology (QUANOMET)” initiative within the project NP-3 “Modell-Nanopartikel”. We also thank Dr. Alaa Edin Gad and Prof. Carsten Ronning for useful discussions on this topic. Furthermore, we gratefully acknowledge Angelika Schmidt and Juliane Breitfelder for support in the synthesis of ZnO nanowires and contacts fabrication.

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(a) Chemical structures of used precursors for the synthesis of different types of C-Dots. (b) Schematic drawing of the ZnO nanowire sensor device functionalized with C-Dots. (c) SEM image of pristine ZnO nanowires (inset: SEM image with higher magnification), (d) SEM image of ZnO nanowires after functionalization with C-Dots (solvent: ethanol) (inset: SEM image with higher magnification), and (e) I-V characteristics of NW devices before and after functionalization. 174x217mm (96 x 96 DPI)


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Normalized absorbance of C-Dots and their corresponding amine precursors measured in suitable solvents (water or ethanol) in the optical range from 250 to 600 nm (a-c). (a) CDs-Ref in grey and PEI-EC in black, (b) CDs-1 in blue and 2,3-DAP in black, (c) CDs-2 in red and 2,3-DAN in black. The CDs-Ref and PEI-EC are measured in water, while CDs-1 and CDs-2 with corresponding amines are measured in ethanol due to different solubility (d) Magnified plot of normalized absorbance (logarithmic scale) of synthesized C-Dot solutions (CDs-Ref in grey, CDs-1 in blue, CDs-2 in red) demonstrates a redshift of the absorption maxima in the spectral energy range from 3.5 to 2.1 eV. 169x225mm (300 x 300 DPI)



ATR-FTIR transmittance spectra of synthesized C-Dots and corresponding amine precursor molecules: (a) CDs-Ref (b) CDs-1, (c) CDs-2. (d) vibrational modes of –C=O group (marked with *) in the range 15002000 cm-1 in different C-Dots: CDs-Ref (grey line), CDs-1 (blue line), CDs-2 (red line) relative to the position of the –C=O vibrations in citric acid (green line). 172x232mm (300 x 300 DPI)


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Hypothetical structure of C-Dots. The region highlighted in red indicates sp2-carbon structure, also referring as a “core” which is surrounded by attached to it various chemical moieties and referred as a “surface”. 70x108mm (150 x 150 DPI)



(a) Exemplary current curve (I-Idark) of bare and functionalized ZnO NW devices recorded during 20 s of illumination at 2.75 eV (~ 450 nm) excitation. Note that for clarity reasons the raw data of bare and functionalized with CDs-2 ZnO NWs were multiplied by a factor 100 and 10, respectively. (b) Obtained photocurrent (current amplitude) of studied nanowires treated with CDs-1 and CDs-2 after 20 s illumination over whole excitation range. (c) The reference measurement of the ZnO NWs device treated with ethanol. 171x225mm (300 x 300 DPI)


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Proposed physical model of an electron transfer from the C-Dots to the ZnO NWs: (a) Hypothetical alignment of the HOMO-LUMO energy levels in C-Dots relative to the energy positions of VBM and CBM in ZnO NW, (b) Position of the HOMO-LUMO energy levels in C-Dots relative to the distribution of DOS in ZnO NW. The blue and red colours correspond to the theoretical HOMO-LUMO levels of CDs-1 and CDs-2, respectively. Here: electron transfer (ET), valence band (VB), conduction band (CB), valence band maximum (VBM) and conduction band minimum (CBM). The arrangement of the HOMO-LUMO levels of the C-Dots was estimated according to the previous studies.19,40 173x77mm (139 x 139 DPI)



TOC Graphic 87x48mm (300 x 300 DPI)


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Shifting the Photoresponse of ZnO Nanowires into the Visible Spectral

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