Oriented Klockmannite CuSe Nanoplates: Polylol Solution Synthesis

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Oriented Klockmannite CuSe Nanoplates: Polylol Solution Synthesis and Its Application on a Inorganic-organic Hybrid Photodetector Jian Wang, Huiming Ji, Junyun Lai, Rongsen Yuan, Xuerong Zheng, Hui Liu, and Zhengguo Jin Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00734 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 4, 2016

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Crystal Growth & Design

Oriented Klockmannite CuSe Nanoplates: Polylol Solution Synthesis and Its Application on a Inorganic-organic Hybrid Photodetector Jian Wang, † Huiming Ji,† Junyun Lai,† Rongsen Yuan,† Xuerong Zheng,† Hui Liu,‡ Zhengguo Jin*, † †

Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education,

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P. R. China ‡

School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300401,

P.R. China KEYWORDS: copper selenide, solution process, nanoplates, CuClSe2, optoelectronic property, photodetector

ABSTRACT: Single-phase and (0001)-oriented hexagonal klockmannite CuSe nanoplates were successfully synthesized through hot injection using green triethylene glycol (TEG)-based solution. It was found that polyvinylpyrrolidone (PVP) as assisting agent played a key role in determining the plate-shaped morphology and hexagonal phase of the CuSe product, and pure Cu2-xSe nanocrystals were obtained without PVP addition under the same other synthetic parameters. The observation to the evolution process with time revealed that CuSe nuclei

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originated from piecemeal decomposing of the CuClSe2 intermediate precursor in the presence of appropriate triethylenetetramine (TETA) reducing agent, and were coalesced in an orientated attachment mechanism. Element Cu constantly kept monovalent state in the whole synthetic process. A new inorganic-organic CuSe nanoplates-poly(3-hexylthiophene) (P3HT) hybrid heterojunction photodetector was first fabricated on silicon substrates, exhibiting a much better photoswitching performance with a low off-current of ~0.14 nA at bias voltage 3.0 V and on/off ratio of >30 under incident light intensity of 100 mW/cm2.

1. INTRODUCTION As an important field of nanostructured inorganic semiconductors, metal chalcogenides have been widely applied in photodetectors, photocatalysis, solar cells, electroluminescent devices, sensors, thermoelectric devices, Li-ion batteries and supercapacitors.1-3 In this regard, copper selenides have attracted much attention due to their unique photoelectric properties suitable for various electronic and optoelectronic applications.4,5 Copper selenides, with typical anisotropic p-type semiconductor characteristics, exist in a wide range of variable stoichiometric phases, including CuSe, Cu2Se, CuSe2, Cu3Se2, Cu5Se4, Cu7Se4, and non-stoichiometric forms of Cu2− xSe.

6,7

Among them, CuSe takes on monoclinic, orthorhombic and hexagonal crystal structure8

and narrow band-gap energies with both an indirect and a direct band gap in the range of 1.0-1.4 eV.9 CuSe exhibits a high electrical conductivity10 and excellent electrogenerated chemiluminescence.11 Particularly, nanostructured CuSe has also been applied as a binary subgroup precursor compound for the synthesis of ternary and above metal selenides, such as leading chalcopyrite CuInSe2-based materials.12


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Nowadays, several solution methods, such as one-pot heating-up method,13 template-directed reaction,14 seed-mediated techniques15 and hot-injection method,16,17 had been used in synthesis of nanocrystals with various nano-shaped growing morphologies, including nanoparticles, nanoplates, nanocubes, nanorods, nanopolyhedra and nanowires. Additionally, the synthesis and shape control of two-dimensional (2D) layered CuSe nanostructures, with the rise of graphene, had recently attracted increasing interest due to their unique anisotropy in conductive property, exceptional optoelectronic and catalytic characteristics.[18-21] The 2D-CuSe nanoflakes had been synthesized by a hydrothermal method using a special Se-containing ionic liquid precursor 1-nbutyl-3-ethylimidazolium methylselenite ([BMIm][SeO2(OCH3)]), and formation mechanism was investigated.22 The 2D-CuSe nanosheets were also prepared through a relatively expensive oleylamine-based solution synthesis, which can be utilized to prepare Cu2-xSe without damaging the shape of the original 2D nanosheets by additional adding Cu+ in reaction solution.18 An oleylamine-based microwave-assisted method was used to prepare CuSe nanosheets with a wide distribution of the lateral size by reaction time of 1 h.10 Green and cheap solvent medium synthesis has had strong attraction to low environmental pollution, less toxic, inexpensive fabrication cost, easy to extraction and purification as well as scalable fabrication in synthetic techniques. So, it is still desired that an eco-friendly solution method was used to prepare 2D-layered CuSe nanostructures. Furthermore, it is a additional difficult for CuSe solution synthesis that single-phase CuSe could be obtained without any other binary phase forms, which was because of its complicate klockmannite structure and intrinsic variable valent states of Cu and Se, when compared with Cu2-xSe solution synthesis.17, 23 The klockmannite crystal structure is consisted of alternating covalently bonded CuSe3-Cu3Se-CuSe3 layers with Se-Se van der Waals layers along the z axis.18 However, the bond valence of Cu with


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Se on all lattice sites is still unclear at present.19 Therefore, the single-phase formation of stable hexagonal klockmannite CuSe is difficult and sensitive to reducing valence of Cu and Se in solution process. Inorganic-organic hybrid photodetectors have many advantages in fabrication techniques and device performance, benefiting from their both the superior intrinsic carrier mobilities, the high, broad-band absorption and light response of the inorganic partners, as well as easy-fabrication, adjustable component compatibility and adequate flexibility.24,25 P3HT, as a important photoelectronic polymer, in conjunction with different inorganic components such as CuInSe2 nanoparticles,25 Cu2ZnSnSe4 nanoparticles,26 Ge nanoparticles,27 CdSe nanowires,24 SnSe Nanosheets28 and Graphene,29 had been developed to construct promising photoelectric devices. However, to the best of our knowledge, the investigation into inorganic-organic hybrid photodetectors with high electrical conductivity 2D CuSe nanoplates (NPs) as inorganic component partner is still not reported so far. In this paper, we develop a new, green and cheap TEG-based solution medium synthesis under ambient pressure condition to prepare single-phase 2D-CuSe NPs. The synthetic process used copper (II) chloride and selenium powder as precursors, TEG as solvent, PVP and TETA as assisting agents. The influence of PVP and TETA adding amounts were investigated. The formation mechanism and the element valences of hexagonal CuSe nanoplates were discussed based on observation of the nucleation and growth with time during synthetic reaction process. The CuSe nanoplates synthesized were used to fabricate an oriented CuSe NPs-P3HT Hybrid film photodetector, which exhibited a much better photoresponse performance.


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2. MATERIALS AND METHODS 1. Materials. Copper(II) chloride (CuCl2·2H2O, 99%), selenium powder (Se, 99%), triethylene glycol (C6H14O4, TEG, 99%), triethylenetetramine (C6H18N4, TETA, 98%), polyvinylpyrrolidone (PVP, K30), absolute ethanol (CH3CH2OH, 99.7%), 1, 2-dichlorobenzene (C6H4Cl2, 99.8%), poly(3-hexylthiophene) (P3HT, 95%, Mw, 30000) and high-purity nitrogen gas. All chemicals were purchased commercially and were used as received. 2. Synthesis of CuSe Nanoplates. 2.1. Typical synthesis procedure. 1 mmol CuCl2·2H2O was dissolved into 10 mL of TEG in a beaker with magnetic stirring at room temperature as copper precursor solution. 1 mmol Se and 0.2220 g PVP (2 mmol, in terms of repeating unit) were poured into 40 mL of TEG solvent in a 100 mL three-necked round-bottom flask as Se precursor solution, which was settled in a heating mantle and connected to a reflux condenser. The flask was heated to the solution temperature of 240 °C where the copper precursor solution was quickly injected into the Se precursor solution with vigorous stirring, and then 80 µL TETA was quickly injected into the Cu-Se reaction solution at temperature of 220 °C. The reaction solution was refluxed at 220 °C for 30 min, and then was cooled to room temperature by water-bath quenching. The synthesized products were extracted by high speed centrifugation and then were purified by washing with excess amounts of absolute ethanol, followed by high speed centrifugation for five times. The purified products were dispersed in ethanol or 1, 2-dichlorobenzene for further characterization and use. 2.2. PVP adding amounts. In a set of syntheses, PVP was added in selected amounts of 1 mmol, 2 mmol and 4 mmol, respectively, while the other processing parameters used were kept the same as those in the typical synthesis above.


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2.3. Investigation of nanoplates growth. In typical synthesis condition, a series of intermediate products were collected at the fixed refluxing temperature 220 °C with different time points of synthetic reaction: before TETA injection, refluxing time of 0 min, 1 min, 3 min, 5 min, 10 min and 20 min after TETA injection, respectively. The synthetic reaction was ended by quickly diluting and cooling the collected reaction solutions with excess amounts of additional TEG solvent. 2.4. TETA adding amounts. In a set of syntheses, TETA adding amounts into reaction solutions were altered by 0 µL, 40 µL, 80 µL and 160 µL, respectively. The other processing parameters used were kept the same as those in the typical synthesis above. 3. Fabrication of Photodetector Devices. P3HT (75 mg) was dissolved in 5 mL 1, 2dichlorobenzene. 200 µL 1, 2-dichlorobenzene solution of CuSe nanoplates (30 mg/mL) was mixed with 100 µL P3HT solution to form a hybrid solution. Comb-like pair electrodes of photodetectors were fabricated on clear SiO2/Si substrates by mask sputtering. The comb-like pair electrode wirings were 30 µm in width and 350 µm in length, respectively. The gap distance between the electrode wirings was 30 µm. CuSe/P3HT hybrid solution was coated on the SiO2/Si substrates by blade-coating method and then was dried naturally in air to prepare CuSe/P3HT hybrid solid photodetector devices. 4. Characterization. X-ray diffraction (XRD) was detected by a D8 advanced X-ray diffractometer (Bruker, Germany) with CuKα radiation (λ=1.5418 Å) at a scan rate of 8°/min in the 2θ range of 10-80°. Hitachi S-4800 field emission scanning electron microscope (Hitachi, Japan) with EDX attachment was used to characterize growing morphology and chemical stoichiometry of CuSe nanoplates. The observation of transmission electron microscopy (TEM),


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high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy dispersive X-ray fluorescence (EDX) analyses were performed using Tecnai G2 F20 field emission transmission electron microscope (FEI, Netherlands). Raman spectra were recorded between 100 cm−1 and 600 cm−1 by a micro-Raman instrument (HR800, HORIBA Scientific) with excitation laser line at 514 nm. UV-Vis–NIR spectra at wavelength from 400nm to 1600nm were collected with a UV-Vis–NIR spectrophotometer (UV-3600, Shimadzu, Japan). XPS was carried out on a PHI 5000 Versa Probe X-ray photoelectron spectroscope (ULVACPHI, Japan). The thickness of CuSe nanoplates was measured by a Multimode nanoscope IV atomic force microscope (AFM, 5500, Agilent, USA). I–V characteristics of the devices were measured at room temperature in air with a Keithley 2400 digital sourcemeter (USA) both in a dark chamber and under illumination at 100 mW/cm2 using an AM 1.5 solar simulator (XES40S1, SAN-EI Electric, Japan). 5. Theoretical details of calculation.The theoretical calculations of surface energies were performed using the DMol3 module of Materials Studio software developed by Accelrys Inc.30 The full geometry optimizations were performed using generalized gradient approximation (GGA) with the Perdew, Burke, and Ernzerhof (PBE) functional and double numerical plus polarization (DNP) basis set.31 The effective core potentials (ECP) were employed for Cu and Se atoms. In all the calculations, we used an 8 × 8 × 8 k-point mesh for the bulk crystal and an 8 × 8 × 1 k-point mesh for the slabs. Real-space global cutoff radii were set at 4.0 Å for all elements. The convergence criterion for self-consistent field (SCF) energy and displacement were set to 1 × 10-5 Hartree and 5 × 10-3 Å, respectively. Thermal smearing of 0.001 Hartree was used in geometry optimizations of slab models. All slabs were separated by a vacuum layer of 15 Å. The calculated lattice constants a=4.078 Å and c=17.696 Å are in good agreement with the


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experimental constants a=3.941 Å and c=17.241 Å, indicating that the selected exchange correlation functional is appropriate. We applied the slab method to determine surface energies. The surface energies γ were obtained by γ= 1/2(Eslab – nCuSeEb)/A where Eslab is the total energy of the specified slab, nCuSe is the number of CuSe units in the slab, Eb is the bulk energy per unit cell with a formula Cu6Se6, and A is the area of the supercell. The factor 1/2 represents that the two surfaces of the slab were taken into consideration. In these calculations, the (001) and (110) surfaces were both modeled with 2 × 2 supercell slabs. For the optimizations of these two slab models, all atomic positions were fully relaxed. Each of these slab models contains 24 Cu atoms and 24 Se atoms. 32-34 3. RESULTS AND DISCUSSION 3.1. Basic Characteristics of CuSe NPs X-ray diffraction pattern of the products synthesized with the typical synthesis procedure is shown in Figure 1a. It is seen that all diffraction peaks of the XRD pattern can be well indexed to the standard pattern of klockmannite CuSe (JCPDS Card No. 65-3562, space group P63/mmc), and no other crystalline phases are detected in the products, suggesting that the as-grown products were single-phase and well-crystallized α-CuSe. The lattice constants calculated based on crystallographic parameters of the hexagonal unit cell are a = b = 3.941 Å, c = 17.241 Å. In addition, the diffraction peak (006) of the XRD pattern has an obvious preferential strengthening


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compared with the standard pattern, which is associated with its growing morphology by SEM observation mentioned below. Figure 1b is the room temperature Raman spectrum of CuSe products. It is known that the klockmannite structure has four bands within the range from 100 cm−1 to 600 cm−1 at room temperature. The strongest peak at 261.6 cm-1 is attributed to stretching modes of the Se–Se pairs. From previous reports, the characteristic Raman frequencies of CuSe are 192 cm-1, 206 cm-1, 263 cm-1 and 525 cm-1, respectively.35 Compared with these results, the four peaks of the CuSe products are all shifted towards lower frequency, which is probably due to the effect of small size and high surface area.36 On the other hand, the SEM image of the products, as depicted in Figure 1c, shows plate-shaped CuSe nanocrystals with lateral size of 265 ± 47 nm and nanoplate thickness of 31.2 ± 3.3 nm measured by AFM (Figure S1). Figure 1d is a HRTEM image of CuSe nanoplates, displaying that the nanoplate is a single-crystalline structure and interplanar distance of 0.340 nm for the ( 1010 )

plane of CuSe. The selected area electron diffraction (SAED) pattern with incident electron

beam perpendicular to the plate-surface shows single-crystalline diffraction spots with typical hexagonal symmetry for hexagonal close-packed lattice (Figure 1e). It demonstrated that the plate-surface of CuSe NPs is [0001] orientation, which is well agreement with the preferential (006) peak strengthening of Figure 1a. EDX spectrum of Figure 1f, corresponding to Figure 1d, shows that the Cu/Se atomic ratio is 1:1.05, being close to the chemical stoichiometry of CuSe compound. In addition, elements C, O and Ni are observed in Figure 1f. The Ni and C are mainly from the nickel grid, and the O is attributed to the absorbed oxygen.


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3.2. Effect of PVP Concentration on the Nanocrystals Growth Figure 2(a-d) is SEM images of the products synthesized with different PVP adding amounts at 220°C for 30 min. It is observed from Figure 2a that the products synthesized without PVP addition are composed of near-granular polyhedral shaped nanoparticles and nanorods. When PVP was added, the synthesized products grew into plate-shaped nanocrystals. Figure 2(e) presents average lateral size, plate thickness and lateral size/thickness ratio vs. PVP adding amounts. The average lateral size of the plate-shaped nanocrystals synthesized with PVP adding amount of 1 mmol, 2 mmol and 4 mmol is 188 nm, 265 nm and 307 nm, respectively, meanwhile the average plate thickness of these samples is 39.4 nm, 31.2 nm and 20.1 nm, respectively, indicating an effective increase of the lateral size/thickness ratio with the PVP adding amount (Figure S1). These results may be explained by an adsorbing and capping effect of large molecule PVP to specific plane (0001) of CuSe nanoplates, which strongly restricted its epitaxial growth on the specific facet. Figure 2f shows XRD patterns of the products synthesized with PVP adding amounts of 0 mmol, 1 mmol, 2 mmol and 4 mmol, respectively, at 220 °C for 30 min. It is seen that the products synthesized without PVP addition are single-phase cubic Cu2-xSe according to JCPDS Card No. 06-0680, which corresponds to the growing morphology of near-granular polyhedral shaped nanoparticles in Figure 2(a). At PVP adding quantity of 1 mmol, the diffraction peaks are a mixed phase which is composed of predominant hexagonal CuSe phase, Cu2−xSe and cubic CuSe2 impurity phase. When PVP adding content is 2 mmol, single CuSe phase is obtained and not changed until to PVP content of 4 mmol. The result indicates that PVP addition actually participated in formation of hexagonal CuSe phase and PVP adding content of 2 mmol is necessary for formation of single-phase CuSe.


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Furthermore, the PVP-assisted function to formation of hexagonal CuSe nanoplates can be analyzed based on solubility-product concept. In synthetic process, the following chemical reactions could take place: a) Formation of Cu2-xSe in absence of PVP. (2 − ) + →

(1)

b) Formation of CuSe in presence of PVP. + →

(2)

where the valence of copper is as +1 and the average oxidation state of selenium is as -1 (see next section). The solubility-product equations involving in the above reactions are expressed as follows:

[ ] =

() [ ]

, [ ] =

( ) [ ]

(3)

where the solubility-product constant Ksp is 7.94×10-49 and 1.58×10-61 for CuSe and Cu2Se, respectively.14 So, we can presume that in the situation of [Cu+] = 1 M, the [Se-] required is 7.94×10-49 M for CuSe precipitation, and the [Se2-] required is 1.58×10-61 M for Cu2-xSe precipitation. Obviously, Cu2-xSe precipitation is easier than CuSe precipitation in the presence of Se2-. TETA strong reduction to free Se atoms at high temperature provided the active Se2- ions to form Cu2-xSe nanocrystals in the absence of PVP, which led to the formation of CuSe2 to balance deficient content of Cu+ in reaction system. In the presence of PVP, such as adding 2 mmol and above, the TETA strong reduction was substantially depressed by carbonyl groups


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complexing of PVP to free Se atoms, which provided the active Se- ions rather than active Se2ions to promote the formation of single-phase CuSe. Figure S2 show HRTEM images and EDS spectra of the CuSe NPs synthesized with PVP adding amounts of 1 mmol, 2 mmol and 4 mmol, respectively. The plate-surfaces of the CuSe NPs with different PVP addition are all [0001] orientation. The Cu/Se atomic ratio with different PVP addition is 1.01:1, 0.95:1, and 0.96:1, respectively. The results indicate that the change of PVP adding amounts had no effect on the preferred oriented growth and the Cu/Se stoichiometric ratio of hexagonal CuSe phase. 3.3. Growth of CuSe NPs with Refluxing Time Growth of CuSe nanoplates in the TEG based solution was investigated. A series of intermediate products were collected at a fixed refluxing temperature 220 °C with different time points of synthetic reaction: before TETA injection, refluxing time of 0 min, 1 min, 3 min, 5 min, 10 min and 20 min after TETA injection, respectively. Figure 3a presents XRD patterns of the intermediate products. Before injecting TETA, the diffraction peaks can be assigned to monoclinic CuClSe2 (JCPDS Card No. 23-0203) and metastable monoclinic CuSe (JCPDS Card No. 49-1456), and the monoclinic CuClSe2 is predominant phase. It is known that CuClSe2 molecule is composed of Cu+ and “neutral selenium molecules” embedded in matrix of Cu(I) chloride.37 In this synthesis, Cu2+ as precursor ions could be reduced to Cu+ by TEG reducing function at high temperature.38 The EDS spectrum shows Cu: Cl: Se atomic ratio of 1: 0.92: 2.62 which is highly Se enrichment(Figure S3a). At refluxing time of 0 min, i.e. just after TETA was injected, the diffraction pattern shows hexagonal CuSe (JCPDS Card No. 65-3562) and cubic CuSe2 (JCPDS Card No. 71-0047). The result indicates that TETA accelerated Se0 reduction to


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Se- and then growth of hexagonal CuSe NPs. By refluxing time of 1 min, the diffraction peaks of hexagonal CuSe appear clearly and that of cubic CuSe2 phase almost disappear. With further extending refluxing time, only the diffraction peaks of hexagonal CuSe present on XRD patterns with refluxing time of 3 min, 5 min, 10min, 20 min and 30 min. Figure 3b is the change of the full width at half maximum (FWHM) of the (0006) peak of hexagonal CuSe with different refluxing times, clearly indicating improvement of the crystallinity by refluxing. Figure 3c is the Cu, Se atomic percentages and the corresponding Cu/Se atomic ratios of the intermediate products and final CuSe nanoplates, which were given by the EDS results, as shown in Figure S3(b–h). The Cu/Se atomic ratio is 0.52:1, 0.70:1, 0.79:1, 0.92:1, 0.93:1, 0.94:1 and 0.95:1 for refluxing time of 0 min, 1 min, 3 min, 5 min, 10 min, 20 min and 30 min, respectively, indicating a gradual evolution of chemical stoichiometry from Cu-poor phase to 1:1 phase. Combined with the above XRD patterns, it is concluded that the synthesized products underwent a way from Cu-poor hexagonal CuSe phase to stoichiometric hexagonal CuSe phase during refluxing process. Figure 4 is the SEM morphologies of the intermediate products and final CuSe NPs collected at 220 °C with refluxing time points of before TETA injection, 0 min, 1 min, 3 min, 5 min, 10 min, 20 min and 30 min after TETA injection, respectively. For the sample before injecting TETA, the SEM image shows predominant rod-shaped precipitates and hexagon-shaped microcrystals, which should be CuClSe2 and metastable monoclinic CuSe compounds, respectively, according to the above XRD result (Figure 4a). For the samples after injecting TETA, the growing morphologies all show plate-shaped nanocrystals (Figure 4b-h). Figure 3d gives that the mean lateral size and standard deviation of the plate-shaped nanocrystals is 185 ± 41 nm, 207 ± 63 nm, 229 ± 51 nm, 230 ± 50 nm, 257 ± 50 nm, 260 ± 48 nm and 265 ± 47 nm respectively at refluxing


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time points of 0 min, 1 min, 3 min, 5 min, 10 min, 20 min and 30 min after TETA injection, which can also be seen in Figure S4. These results demonstrate that the TETA-assisted TEG/PVP solution synthesis had a rapid plate-shaped nanocrystals growth. Original plate-shaped nanocrystals with average lateral size of 185 nm could be quickly formed once TETA was injected. And subsequently the average lateral size was further increased by 11.9%, 23.8%, 24.3%, 38.9%, 40.5% and 43.2% respectively at refluxing times of 1 min, 3 min, 5 min, 10 min, 20 min and 30 min, accompanying with a decreased standard deviation in the lateral dimension. The quick growth feature, i.e. becoming slow growth rate and becoming small standard deviation, was related to Cu and Se precursor consumption as well as the well-known Ostwald ripening with extending of refluxing time.39,40 Figure 5a shows the typical original nanoplate morphology with irregular outline at refluxing time of 0 min. It clearly demonstrates that dominant growth of the plate-shaped nanocrystals should depend on the oriented attachment mechanism at the early stage of refluxing. The HRTEM image of area A marked in Figure 5a displays the coherent region between crystallite I and crystallite II through spontaneous self-organization of adjacent small particles to reduce overall surface energy (Figure 5b).41 PVP large molecules may play a crucial role in the oriented attachment growth by capping the (0001) plane surfaces selectively to promote its two dimension growth.42 Selected-area electron diffraction pattern of the individual nanoplate reveals that it is monocrystalline klockmannite structure with [0001] orientation and pronounced ( 1010 ) and ( 0110 ) reflection, indicating its single lattice orientation and perfected coherent boundaries by selforganized attachment of those primary nanocrystals during the oriented attachment growth ( Figure 5c).


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The chemical valences of element Cu and Se of the specific products collected at three refluxing time points, i.e. before TETA injection, 0 min and 30 min after TETA injection, were analyzed by XPS. Table 1, based on Figure S5, lists the binding energy peak positions of Cu2p3/2, Se3d and O1s regions for the collected products. The binding energy of Cu2p3/2, Se3d and O1s of the quenching products at before TETA injection is 932.3 eV, 55.3 eV and 531.0 eV, respectively, indicating that element Cu and Se valence is Cu+ and Se0, and mainly corresponds to the formed CuClSe2 phase.43,44 For the O1s peak at 531.0 eV, it is partly attributed to carbonyl (C=O) oxygen atoms from the adsorbed PVP molecules on the products. Compared with that of pure PVP molecules, the O1s peak shifts from 531.3 eV to 531.0 eV.45 The slight shift illustrates that the electron density increases around the carbonyl oxygen atoms due to interaction among PVP molecules and Se0 cores. And meanwhile the Se3d peak shifts toward higher binding energy of 55.3 eV when compared with that of pure Se phase(54.9 eV), which is also well consistent with the situation that chemical activity of Se0 is depressed by interaction with carbonyl groups of PVP molecules. At 0min after TETA injection, the binding energy of Cu2p3/2, Se3d and O1s of the quenching products is 931.7 eV, 53.9 eV and 531.7 eV. The former both respectively correspond to monovalent Cu state and average Se- valence state in the selenium-rich hexagonal CuSe lattice sites. The O1s binding energy of 531.7 eV is still higher than that of pure PVP molecules, possibly indicating that PVP molecules intensified to bind with Cu+ cores on the capping crystallographic facets of the formed CuSe nanoplates by carbonyl groups. Up to 30min after TETA injection, the binding energy of Cu2p3/2, Se3d and O1s of the final stoichiometric CuSe phase formed are essentially unchanged when compared with that of the quenching products at 0 min after TETA injection, suggesting that element Cu kept


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constantly monovalent state in the whole synthetic process, and element Se held average Sevalence state to equilibrate the 1:1 chemical stoichiometry and electric neutrality with monovalent Cu+ ions. Table 1. XPS binding energy peak positions on the basis of Figure S5

no TETA

0 min after

30 min after

injection

injection

injection

Cu2p3/2

932.3 eV

931.7 eV

931.6 eV

Se3d

55.3 eV

53.9 eV

54.0 eV

O1s

531.0 eV

531.7 eV

531.7 eV

peak

3.4 Growth Mechanism of CuSe NPs Based on above results, a possible formation mechanism of the CuSe NPs by TEG-based solution synthesis is sketched in Scheme 1. The synthesis process may involve in the six aspects as follows: (i) Not injecting copper precursor solution. In this stage, as the Se-containing and PVPcontaining solution temperature reached the melting point of selenium, the solution changed into transparent bright orange colour, in which PVP large molecules might catch free Se atoms by carbonyl groups coordination to form a PVP-Se complexes. The carbonyl groups coordination to free Se atoms actually passivated chemical activity of the Se atoms, which was benefited to form single-phase CuSe NPs. Particularly, it is noticeable that the single-phase CuSe NPs can be


16

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obtained only when PVP and Se mole ratio equals to 2 and above (Figure 2f), implying that each Se atom might need at least two carbonyl groups to reach the matched coordination to depress Se reduction, and then to provide active Se- ions rather than Se2- ions for formation of the singlephase CuSe NPs. So, it is concluded that PVP has an essential role not only as a capping agent for two dimension growth of CuSe NPs but also as a complexing agent of free Se atoms for formation of single-phase hexagonal CuSe. (ii) Injecting copper precursor solution. When copper precursor solution was injected, the solution changed colour from bright orange to brown. The quenching products have been verified to be main phase CuClSe2 as well as metastable monoclinic CuSe second phase, in which the element Cu involved in both reduced its valence from Cu2+ precursor to Cu+ resultants by reducing ability of TEG hydroxyl groups at high temperature (Figure 3a). The reaction equations related to form CuClSe2 are as follows: !"

#$$%

Cu + Cl- + 2Se, → CuClSe

(4)

(5)

In our experiments, it had been found from Figure S6 that large-sized metastable monoclinic CuSe NPs could be formed under this condition at 220 °C for 30 min by gradual CuClSe2 precursor consumption and slow Se release from PVP-Se and then Se reduction from Se0 to Sealso by TEG hydroxyl groups. (iii) Injecting TETA into reaction solution. TETA has much strong reducing ability at high solution temperature, which actually accelerated Se reduction of CuClSe2 precursor from Se0 to Se- so that CuClSe2 precursor was piecemeal decomposed into selenium-rich hexagonal CuSe1+x


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nuclei. Scheme 1 illustrates the lattice parameters of monoclinic CuClSe2 and hexagonal klockmannite CuSe. The crystal structure of CuClSe2 is built up of pseudo-fourfold screws of Se atoms along y axis. The screws are connected by Cu-Cl-Cu links. Each copper atom is tetracoordinated, which is linked with two Se and two Cl forming a slightly distorted tetrahedron.46 In klockmannite CuSe crystal structure, one third of the Cu atoms are coordinated to three Se atoms in a triangle, whereas the remaining Cu atoms are surrounded by four Se atoms to form a tetrahedron. From a supercell model of klockmannite CuSe (Scheme 1c), it has been found that the CuSe crystal structure consists of alternating CuSe3-Cu3Se-CuSe3 layers and Se-Se layers along the z axis. Furthermore, the interactions within the CuSe3-Cu3Se-CuSe3 layers are covalent bonds, whereas the interactions between the Se-Se layers are van der Waals forces. The nearestneighbor interatomic distances and bond angles in CuClSe2 and CuSe are summarized in Scheme 1. We find out that the similarity of bond distance and angles between CuClSe2 and CuSe crystals supplies a possible route that leads to transform directly from CuClSe2 unit to CuSe unit by breaking the weak Se-Se bond (red arrow) in the screws for nucleation of selenium-rich hexagonal CuSe nuclei. The following reaction equation (6) presents the piecemeal decomposition of CuClSe2 into selenium-rich hexagonal CuSe1+x nuclei, in which Se0 in CuClSe2 was partly reduced into Se- at 220 °C mainly by TETA. -.-/

CuClSe #$$$% CuSe01 nuclei + (1-x)Se- + Cl- (x < 1)

(6)

The effects of TETA adding amount, at 40 µL, 80 µL and 160 µL, on single-phase formation of CuSe NPs was probed by SEM and XRD, as shown in Figure S6. It is seen from the XRD patterns that the single-phase CuSe can be obtained in the range of 40 µL-80 µL. At 160 µL, the diffraction peaks present cubic Cu2−xSe and cubic CuSe2 phases, except for hexagonal CuSe


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phase. These results suggested that the excess TETA amount speeded up Cu and Se reduction, which resulted in the excess Cu+ reacted with Se2- to form Cu2−xSe phase and meanwhile reacted with (Se2) – to form CuSe2 phase. (iv) Preferential 2-dimension nucleation and growth. When selenium-rich hexagonal CuSe nuclei were formed, the growing shape of the primary CuSe nuclei was dominated by its characteristic unit cell structure.47-49 According to ab initio calculation results of structure optimization for hexagonal CuSe, the surface energies (γ(100)/(010)) of the lateral facets (100) or (010) is 1.12 J/m2, which is larger than the surface energy (γ(001))of the planar (001) facet (0.91 J/m2). So, this surface energy difference results in faster crystal growth rate in the lateral directions due to spontaneous decrease of intrinsic energy. Furthermore, when the planar (001) facets grew into enough large size, the polymer-sized PVP molecules were easy to cover effectively on the (001) surfaces of the formed CuSe nanocrystals, which further enlarged the surface energy difference between the top-bottom facets and the side facets to promote the 2dimension nucleation and growth. The growth reaction can be expressed according to equation (7): CuSe01 nuclei + yCu + (1-x)Se- → Cu09 Se unit (y < 1)

(7)

(v) 2-dimension Oriented attachment growth. With preferential 2-dimension epitaxial growth, the oriented attachment growth among small-sized CuSe as building units developed into a dominate growth mechanism. Figure 5 has illustrated the characteristics of the oriented attachment growth. The large surface energy difference derived from the different crystallographic planes combined with the PVP capping effect supplied enough driving force to trigger the more effective and quicker decreasing way of surface energy.


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(vi) Ostwald ripening. The Ostwald ripening is a well-known spontaneous process concerning energy minimum and energy balance in a solution system based on interface thermodynamic behavior. During the last process, the irregular-sized CuSe NPs developed into more perfectshaped ones partly by the cost of some small CuSe NPs consumption. The crystallinity of the hexagonal CuSe NPs was improved and the average lateral size was increased by 43.2%, accompanying with a decreased standard deviation in the lateral dimension, through refluxing of 30 min. Moreover, the chemical stoichiometry of the hexagonal CuSe NPs formed gradually developed from selenium-rich CuSe phase to stoichiometric CuSe phase. The chemical reaction related to the formation of stoichiometric CuSe NPs is as follows: 0; + (1 − ?) →

(8)

3.5 Application of CuSe NPs to the oriented CuSe-P3HT Hybrid Photodetectors CuSe NPs were mixed with P3HT to prepare an oriented CuSe-P3HT hybrid film photodetector device on SiO2/Si wafer, as shown in Figure 6a and Figure S7, indicating the CuSe nanoplates were packed into lamellar phases in the photovoltaic devices. Figure 6b shows the I-V curves of the hybrid film device, which was measured in the dark condition and under simulated sunlight illumination. It is seen that a large enhancement of photocurrent was observed when the device was illuminated using the simulated sunlight with a power of 100 mW/cm2. In the dark, the device was nearly insulating. Figure 6c is the photocurrent gain and decay as a response to the on/off states by periodically turning the light on and off with an intensity of 100 mW/cm2 at a bias of 3.0 V. In the dark, the bias current was only 0.14 nA, demonstrating an excellent cut-off capability and low noise. At incident light intensity of 100 mW/cm2 and bias voltage of 3.0 V, the current response could approach 4.4 nA, giving an on/off ratio of >30. In contrast, it had been


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reported that the pure P3HT film device had only low photoresponse and the on/off ratio was less than 2.23,50 The low on/off ratio was because that the excitons generated in pure P3HT under light irradiation could not dissociate efficiently into electrons and holes in the absence of the electron acceptor. Also, the photoresponse of the single CuSe NPs film device was measured under the same condition as CuSe-P3HT hybrid film device in our experiments. The dark current and photocurrent was 2.4 nA and 3.7 nA, respectively, giving an on/off ratio of 1.54, which was still much low as that of the pure P3HT film device (Figure 6d). However, the oriented CuSeP3HT hybrid film structure gave an effective electrons-holes dissociation route according to the energy level alignment, as shown in Figure 6a. The direct and indirect band gap of CuSe NPs films was determined to be 1.53 eV and 1.12 eV, respectively, by the Kubelka-Munk function using its diffuse reflectance spectrum. 51 (Figure S8) It is found that the direct band gap energy has blue shift by 0.48 eV due to the 1D nanosize effect relative to bulk CuSe (1.05 eV). Based on the direct band-gap structure, the bottom of the conduction band, which corresponds to the lowest unoccupied molecular orbit (LUMO) of -3.2 eV for P3HT, is -4.51 eV.10 The top of the valence band, which corresponds the highest occupied molecular orbit (HOMO) of -5.2 eV for P3HT, was calculated to be -6.04 eV for CuSe NPs. So, it is suggested that the energy level alignment of the CuSe-P3HT blend shows a main holes process occurring in the hybrid photodetector. The high on/off ratio originates from the band-structure match between both, which builds up an efficient photogenerated charge dissociation at the interface. Upon illumination with photon energies larger than the CuSe semiconductor band gap, electron-hole pairs are photogenerated and the photogenerated holes are readily transferred to the P3HT side at their interface, leaving unpaired electrons behind, which increases the electronic conductivity of CuSe NPs under an applied electric field. On the other hand, P3HT also plays a role on


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electrons-holes photogeneration. The exciton dissociation is well known to occur efficiently at the interface of organic-inorganic semiconductors mixed together in a blend film, such as a conjugated polymer and a fullerene derivative.50,52,53 For charge transport process, the CuSe NPs can act as a photoelectron acceptor, whereas P3HT is perfect for acting as a hole acceptor in the CuSe NPs-P3HT hybrid structure. In addition, it is notable that the off-current of the oriented CuSe-P3HT hybrid film device is ~0.14 nA at bias voltage of 3.0 V, which is far lower than that of single CuSe NPs films (~2.4 nA) at the same bias. The result may be attributed to difficult charge transportation through the both interface without light illumination due to its specific energy level alignment. The similar result is also reported in the other previous study.24 In conclusion, the oriented CuSe NPs were dispersed in P3HT matrix, forming a 3D interconnected network, which exhibits a better photoresponse performance. Its switching in the two states was fast, high sensitive and reversible, allowing the device to serve as a high-quality photosensitive switch. 4. CONCLUSIONS Single-phase and (0001)-oriented hexagonal klockmannite CuSe nanoplates were successfully synthesized by the atmospheric pressure hot injection method. The CuSe nanoplates synthesized was (0001) orientation and had the lateral size of 265 ± 47 nm and plate thickness of 31.2 ± 3.3 nm. The assisting agent PVP played a key role not only in the capping effect for two dimension growth of CuSe NPs, but also in the complexing function to free Se0 atoms for formation of single-phase hexagonal CuSe. In this process, TETA strong reducing ability in synthetic solution was substantially depressed by the PVP-Se complex so that the active Se- ions became dominant. A possible formation mechanism of the CuSe NPs by the TEG-based solution synthesis was suggested. In the early stage of reaction, reducing catalysis of TETA triggered the decomposition


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from CuClSe2 intermediate precursor to selenium-rich hexagonal CuSe1+x nuclei based on the similarity of structure between both. And subsequently, the stoichiometric and oriented CuSe NPs were grown by the oriented attachment and the Ostwald ripening, accompanying with improvement of crystallinity and average lateral size. A new inorganic-organic hybrid film photodetector using the oriented CuSe nanoplates and photoelectronic polymer P3HT as blending partners was fabricated on SiO2/Si wafer, exhibiting a much better photoswitching performance with a low off-current of ~0.14 nA at bias voltage 3.0 V and on/off ratio of >30 under incident light intensity of 100 mW/cm2. Its switching in two states was fast, high sensitive, low noise and reversible, allowing the device to serve as a promising high-quality photosensitive switch.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Supplementary characterizations: AFM, TEM, EDS, XPS, SEM, size distribution histogram and diffuse reflectance spectrum

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS

The authors acknowledge financial support from the Key Natural Science Foundation of Tianjin city (Contract No. 12JCZDJC27500) and National Nature Science Foundation of China (Contract No. 51402085). This research used the resources of National Supercomputing Center in Shenzhen. The authors would also like to express gratitude to Ms. Yajing Han and Dr. Jing Mao for their assistance in SEM and TEM measurement.

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For Table of Contents Use Only Oriented Klockmannite CuSe Nanoplates: Polylol Solution Synthesis and Its Application on a Inorganic-organic Hybrid Photodetector

Jian Wang, Huiming Ji, Junyun Lai, Rongsen Yuan, Xuerong Zheng, Hui Liu, Zhengguo Jin

Single-phase and (0001)-oriented hexagonal klockmannite CuSe nanoplates were successfully synthesized by the atmospheric pressure hot injection method. A new oriented CuSe-P3HT hybrid film photodetector was first fabricated on SiO2/Si wafer, exhibiting a low off-current of ~0.14 nA at bias voltage 3.0 V and on/off ratio of >30 under incident light intensity of 100 mW/cm2.


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Figure 1. (a) XRD pattern, (b) Raman spectrum, (c) SEM image, (d) HRTEM image, (e) SAED pattern and (f) EDX spectrum of the synthesized α-CuSe nanoplates with PVP adding amount of 2 mmol at 220 °C for 30 min. 220x247mm (300 x 300 DPI)



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Figure 2. SEM images of products synthesized with PVP adding amounts of (a) 0 mmol, (b) 1 mmol, (c) 2 mmol and (d) 4 mmol at 220 °C for 30 min. (e) lateral size/thickness ratios of the products with the different PVP amounts and (f) XRD patterns. 220x240mm (300 x 300 DPI)


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Figure 3. (a) XRD patterns of a series of intermediate products collected at 220 °C with different refluxing time points (The diffraction peaks marked by ★, ●, ▲ and ◆ are hexagonal CuSe, cubic CuSe2, monoclinic CuSe and monoclinic CuClSe2, respectively.) (b) The enlarged details around (006) diffraction peaks of the intermediate products. (c) Change of Cu/Se atomic ratios of the intermediate products. (d) Mean size and standard deviation of the intermediate products obtained based on Figure S4. 500x393mm (300 x 300 DPI)



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Figure 4. SEM images of a series of intermediate products collected at 220 °C with refluxing time points of (a) Before TETA injection, (b) 0 min, (c) 1 min, (d) 3 min, (e) 5 min, (f) 10 min, (g) 20 min and (h) 30 min after TETA injection. 220x308mm (300 x 300 DPI)


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Figure 5. Original CuSe nanoplate synthesized at refluxing temperature 220 °C for 0min: (a) TEM image, (b) HRTEM image and (c) SAED pattern. 220x148mm (300 x 300 DPI)



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Scheme 1. The schematic formation process and mechanism of the CuSe NPs 111x72mm (300 x 300 DPI)


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Figure 6. (a) The energy level diagram and device structure of CuSe-P3HT blend: I. Absorption within both components of the layer. II. Electron transfer from the P3HT to the CuSe. III. Hole transfer from the CuSe to the P3HT. (b) I–V characteristics of the CuSe NPs-P3HT hybrid photodetector. Transient photocurrent of (c) the CuSe NPs-P3HT hybrid device and (d) single CuSe NPs films under illumination at 100 mW/cm2 and a voltage of 3 V turned on and off at 10 s intervals. 111x83mm (300 x 300 DPI)


Oriented Klockmannite CuSe Nanoplates: Polylol Solution Synthesis

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