SnO2

Article pubs.acs.org/JPCC

Current Transport Properties of CuS/Sn:In2O3 versus CuS/SnO2 Nanowires and Negative Differential Resistance in Quantum Dot Sensitized Solar Cells Matthew Zervos,*,† Eugenia Vasile,‡ Eugeniu Vasile,‡ Evangelia Karageorgou,† and Andreas Othonos§ †

Nanostructured Materials and Devices Laboratory, School Of Engineering, and §Laboratory of Ultrafast Science, Department of Physics, University of Cyprus, P. O. Box 20537, Nicosia 1678, Cyprus ‡ Politehnica University of Bucharest, 313 Splaiul Independentei, Bucharest, Romania ABSTRACT: The structural, optical, and electrical transport properties of nanowires obtained by the deposition of Cu over Sn doped In2O3 and SnO2 nanowires followed by processing under H2S between 100 and 500 °C have been investigated for their use in quantum dot sensitized solar cells. We find that the CuS/Sn:In2O3 nanowires obtained between 100 and 200 °C consist of hexagonal CuS and cubic In2O3 but higher temperatures lead to the formation of Cu0.23In2.59S4 nanowires. Moreover, we observed the existence of SnO2 quantum dots in tetragonal Cu2SnS3 nanowires obtained at 400− 500 °C which are responsible for ultraviolet emission at 3.65 eV and a breakdown of the dipole forbidden rule in SnO2.The CuS/Sn:In2O3 nanowires obtained at lower temperatures exhibit better rectifying current−voltage characteristics and higher currents, but we did not observe negative differential resistance, as expected from a p−n tunnel junction, although this occurred by bringing Sn:In2O3 nanowires in weak contact with p-type CuS, similar to a cat’s whisker device. We discuss the origin of the negative differential resistance which was also observed in connection with the TiO2 barriers deposited on the transparent conducting oxide anode and its importance for quantum dot sensitized solar cells. on Si(001) by the vapor−liquid−solid (VLS) mechanism4 but also the effect of postgrowth processing SnO2 and Sn:In2O3 NWs under H2S on their structural, electrical, and optical properties.14 In addition, we have showed that Cu2SnS3/SnO2 and CuInS2/Sn:In2O3 NWs may be obtained via the deposition of Cu over SnO2 and Sn:In2O3 NWs followed by postgrowth processing under H2S.15 The Cu2SnS3/SnO2 NWs exhibited ultraviolet photoluminescence (PL) at 340 nm which was attributed to the formation of SnO2 quantum dots (QDs) and a breakdown of the dipole forbidden rule but without confirming the actual existence of the SnO2 QDs. Such Cu2SnS3/SnO2 NWs are interesting for the realization of solar cells using lowcost, nontoxic materials like Cu2ZnSnS4 solar cells whose efficiencies have reached 10%.16 Furthermore, the Cu2SnS3/ SnO2 NWs are similar to the Cu2S/Sn:In2O3 NW core−shell p−n tunnel junctions which have been used as efficient counter electrodes (CEs) in quantum dot sensitized solar cells (QDSSCs) by Jiang et al.17 who decorated Sn:In2O3 NWs with Cu2S QDs by solution processing methods. It should also be noted that p-type Cu2S has a high work function of 5.0 eV,6 making it an attractive alternative to Au and Pt CEs in QDSSCs, while efforts to improve the performance of

1. INTRODUCTION Metal oxide semiconductor nanowires (NWs), such as n-type ZnO,1 SnO2,2 In2O3,3 and Sn doped In2O3 NWs,4 have been investigated extensively in the past and are important for the fabrication of electronic and optoelectronic nanoscale devices, such as nanowire solar cells, sensors, photodetectors, etc.5 On the other hand, p-type chalcogenide semiconductors such as Cu2S have been used in Cu2S/CdS solar cells, which have efficiencies of 9−10%.6 While Cu2S is a low-cost, nontoxic semiconductor, it has been used mainly in conjunction with CdS, but recent efforts on Cu2S/ZnO core−shell nanowire solar cells (NWSCs) suggest that the potential of Cu2S can be exploited further in combination with other metal oxides.7−9 It should be noted that Cu2−xS has different crystal structures and energy band gaps, i.e., CuS (2.2 eV), Cu1.75S, Cu1.8S (1.5 eV), Cu1.95S, and Cu2S (1.2 eV),10 but more importantly it has a metallic like conductivity and it is semi-transparent similar to indium tin oxide (ITO). In addition, Cu2S has been deposited on ITO films11,12 while flexible solar cells of p-type Cu2S NWs in contact with n-type ITO deposited on polyethylene have been demonstrated by Wu et al.13 Consequently the growth of high crystalline quality metal oxide (MO) NWs such as n-type SnO2 or Sn doped In2O3, hereafter noted as Sn:In2O3 NWs, and their combination with p-type semiconductors like Cu2−xS, is attractive for the fabrication of NWSCs. Recently we investigated the structural properties of Sn:In2O3 NWs grown © XXXX American Chemical Society

Received: August 26, 2015 Revised: December 14, 2015

A

DOI: 10.1021/acs.jpcc.5b08306 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C photoanodes (PAs) in QDSSCs are also ongoing. For instance, TiO2 has been deposited by atomic layer deposition over ITO nanoparticles (NPs)18 or over Sn:In2O3 NWs as PAs19 while the importance of recombination at the transparent conducting oxide, liquid-electrolyte interface has also been described by Ruhle et al.20 who showed that a TiO2 layer deposited on ITO can prevent electron hole recombination and increase the overall efficiency. Here we have investigated the electrical, structural, and optical properties of nanowires obtained by the deposition of Cu over Sn:In2O3 and SnO2 NWs followed by postgrowth processing under H2S between 100 and 500 °C and their application as CEs in QDSSCs. We find that the CuS/Sn:In2O3 NWs obtained between 100 and 200 °C consist of hexagonal CuS and cubic bixbyite In2O3 but higher temperatures lead to the formation of Cu0.23In2.59S4 NWs which consist of irregularly stacked crystals with sizes of 60−70 nm. In addition, we confirm the existence of SnO2 QDs with sizes ranging from 2 to 6 nm in tetragonal Cu2SnS3 NWs after the deposition of Cu over the SnO2 NWs and postgrowth processing under H2S between 400 and 500 °C. The SnO2 QDs are responsible for the observation of ultraviolet (UV) emission at 340 nm or 3.7 eV and a breakdown of the dipole forbidden rule. We also find that the CuS/Sn:In2O3 NWs exhibit better rectifying current voltage (I−V) characteristics compared to CuS/SnO2 NW CEs, but we did not observe negative differential resistance (NDR) which occurs in p−n tunnel junctions. Instead we observed NDR with a peak to valley ratio of ≈1.7 at 0.6 V, between ptype CuS and n-type Sn:In2O3 NWs, which were brought into weak contact at room temperature, without the formation of a solid state junction, similar to a cat’s whisker device. More importantly, we have observed NDR in connection with TiO2 barriers having thicknesses greater than 5 nm deposited on ITO as PAs. We discuss the origin of the NDR in each case and its importance for QDSSCs.

Figure 1. Schematic diagram of (a) geometry of CE and optical image of the Sn:In2O3 NWs grown on quartz, (b) selective deposition of 60 nm Cu over the left half of the Sn:In2O3 NWs, (c) resultant Cu/ Sn:In2O3 core−shell NWs, (d) postgrowth processing under H2S and the conversion to CuS/Sn:In2O3 NWs, (e) deposition of 50 nm Au over the right half Sn:In2O3 NWs, (f) spacer over the CuS/Sn:In2O3 NWs and optical image of the finished CuS/Sn:In2O3 core−shell NW CEs, (g) spin coating of TiO2 NPs over TiO2/ITO at 5000 rpm/60 s and formation of CdS/CdSe QDs by SILAR, (h) optical images of CdS QDs on TiO2 NPs showing yellow color after five consecutive SILAR cycles, and schematic of the resultant ITO/TiO2/CdS/CdSe PA (i) section of complete cell. The same process was followed for the fabrication of the CuS/SnO2 core−shell NW CEs.

conditions and substrate geometries described above for the Sn:In2O3 NWs. The morphology of the Sn:In2O3 and SnO2 NWs was initially inspected by scanning electron microscopy (SEM), while their crystal structure was determined by X-ray diffraction (XRD) using a Rigaku Miniflex. Subsequently 40 nm of Cu was deposited by sputtering over the SnO2 and Sn:In2O3 NWs that were grown on Si(001) and only 10 nm Cu over the SnO2 and Sn:In2O3 NWs that were grown on quartz in order to maintain transparency, as illustrated schematically in Figure 1b and c. Both Cu/SnO2 and Cu/Sn:In2O3 core−shell NWs were exposed to a constant gas flow of 50 sccms H2S at 100, 200, 300, 400, and 500 °C for 60 min using a ramp rate of 10 °C/ min after which their morphology was inspected by SEM using a Quanta Inspect F50 with 1.2 nm resolution, equipped with an energy dispersive X-ray spectrometer (EDXS) having a resolution of 133 eV at MnKα, while their crystal structure was determined again by XRD. High-resolution transmission electron microscopy (HRTEM) was carried out using a TECNAI F30 G2 S-TWIN operated at 300 kV with energy dispersive X-ray analysis (EDAX) facility. The CEs were completed via the deposition of a 50 nm Au contact layer as shown in Figure 1d and e. Care was taken to deposit gold on the Sn:In2O3 NWs that were not covered initially by Cu. A 70 μm thick spacer with a 10 mm × 10 mm window was fitted around the active area of the CEs as shown in Figure 1f for the purpose of constructing the QDSSC. The window was positioned only over the active area of the CuS/SnO2 and CuS/Sn:In2O3 NWs, not over the Sn:In2O3 NWs covered with Au. Besides the above, we also prepared ITO/TiO2 PAs decorated with CdS/CdSe QDs as shown in Figure 1g. Initially we deposited 5, 10, 20, and 40 nm of Ti on top of ITO slides which were subsequently oxidized at 500 °C under a constant flow of 200 sccms of O2 for 60 min using a ramp rate of 10 °C/

2. METHODS Sn doped In2O3 NWs were grown via the VLS mechanism using Au as a catalyst on Si(001) and quartz. The Sn:In2O3 NWs were grown at 800 °C and 1 mbar for 30 min using a lowpressure chemical vapor deposition (LPCVD) hot wall reactor, capable of reaching 1100 °C and consisting of a 1″ quartz tube that was fed from four mass flow controllers that can supply Ar, O2, NH3, and H2. More specifically, rectangular samples of Si(001) and quartz with dimensions of ≈15 mm × 25 mm were cleaned sequentially in trichloroethylene, methanol, acetone, and isopropyl alcohol, rinsed with deionized water, and dried with nitrogen after which a layer of ≈1 nm Au was deposited over a 10 mm × 10 mm area as shown in Figure 1a. For the growth of the Sn:In2O3 NWs, Sn (Aldrich, 2−14 Mesh, 99.9%) and In (Aldrich, 99.9%) were weighed with an accuracy of ±1 mg. In particular 0.2 g of In containing 0.01 g of Sn and the substrates were positioned at the center of the 1″ quartz tube, which was pumped down to 10−4 mbar, and subsequently purged with 600 sccḿ s of Ar for 10 min at 10−1 mbar after which the temperature was ramped to 800 °C at 30 °C/min while maintaining the same flow of Ar. Upon reaching 800 °C, a small flow of 10 sccms O2 was added to the main flow of Ar, in order to grow the Sn:In2O3 NWs for 30 min. Finally the quartz tube was allowed to cool down to room temperature while maintaining the same flow of Ar but without O2. Similarly, SnO2 NWs were also grown on Si(001) and quartz via the VLS mechanism using exactly the same growth B


Article

The Journal of Physical Chemistry C min. These conditions were determined from the deposition of 40 nm Ti on Si(001) and quartz which was oxidized between 100 and 900 °C using 200 sccms O2 and a ramp rate of 10 °C/ min after which the crystal structure and transparency of the resultant TiO2 were determined by XRD and steady state absorption transmission spectroscopy using a PerkinElmer 25 UV/vis spectrophotometer. Following this, about 0.3 g of 25 nm TiO2 NPs were dispersed into 1 mL of butanol with a few drops of Triton X and left to stir overnight after which they were spin coated at 5000 rpm for 30 s over the TiO2/ITO slides. The TiO2 NPs were subsequently annealed at 500 °C for 60 min under 200 sccms O2 in order to improve their cohesion. Furthermore, CdS QDs were deposited on the TiO2 NPs by sequential ionic layer adsorption and reaction (SILAR) using 0.01 M Cd(NO3)2 in pure ethanol C2H5OH and 0.01 M Na2S in methanol CH3OH. The CdS QDs were formed after dipping alternately in Cd(NO3)2 and Na2S four consecutive times. An optical image of the TiO2 NPs loaded with CdS QDs is shown in Figure 1h. This was followed by the deposition of a CdSe QD absorber layer via the reduction of SeO2 by NaBH4 in pure ethanol C2H5OH according to the reaction described by Lee et al.21 and a passivation layer of ZnS. Finally a polysulfide electrolyte solution consisting of 1 M S, 1 M Na2S, and 0.1 M KCl in H2O:CH3OH (3:7) was prepared with a pH ≈ 14 which had a characteristic yellow color. The QDSSC cell was assembled by applying dropwise the polysulfide electrolyte in the active area of the CEs after which the PA was positioned over the CE and clamped down onto the spacer, using foam board with a 8 mm diameter circular window. A schematic diagram of the complete cell is shown in Figure 1i, and the current−voltage (I−V) characteristics of the cells were measured in a two-terminal configuration, using a Keithley 2635A, in the dark and under one sun illumination.

Figure 2. (a) SEM images of the CuS/Sn:In2O3 core−shell NWs obtained at 100, 200 °C and Cu0.25In2.5S4 NWs at 300 and 400 °C, (b) TEM and HRTEM of the CuS/Sn:In2O3 core−shell NWs, and (c) EDX spectra obtained from different areas over the Au NP, CuS shell, and In2O3.

3. RESULTS AND DISCUSSION We will begin by describing the properties of (a) the Cu/ Sn:In2O3 and (b) Cu/SnO2 NWs after postgrowth processing under H2S between 100 and 500 °C before considering their current transport properties as CEs in a QDSSC. Properties of CuS/Sn:In2O3 NWs. The Sn:In2O3 NWs that were grown at 800 °C had diameters of 100 nm, lengths up to ≈10 μm, and the cubic bixbyite crystal structure of In2O3 as we have shown in detail previously,4 but also a metallic like conductivity or small resistances of the order of 1−10 Ω due to the very high carrier density of electrons, i.e. 1019−1020 cm−3. A typical SEM image after the deposition of 40 nm Cu over the Sn:In2O3 NWs and postgrowth processing under H2S for 60 min at 100 and 200 °C is shown in Figure 2a. The resultant CuS/Sn:In2O3 NWs exhibited clear and well resolved peaks in the XRD as shown in Figure 3a belonging to the cubic bixbyite crystal structure of In2O3 and hexagonal covelite CuS. The CuS/Sn:In2O3 NWs consist of a thin shell of CuS surrounding the cubic In2O3 core as shown by the TEM and HRTEM in Figure 2b . The CuS shell segments have a maximum thickness of ≈30 nm and lattice spacings of 2.82 and 3.21 Å along the [103] and [101] crystallographic directions, respectively. However, it appears that the interface between the CuS and the In2O3 is not atomically abrupt and the CuS shell does not have a uniform thickness all along the length of the Sn:In2O3 NWs which are bare at certain points and which is important when considering the properties of the CuS/Sn:In2O3 NWs as CEs in QDSSCs. One may also observe the Au NPs on the top of the Sn:In2O3 NWs confirmed by EDX as shown in Figure 2c,

Figure 3. (a) XRD of CuS/In2O3 NWs obtained from Cu/In2O3 NWs under H2S at 200 °C for 60 min; (b) XRD of the Cu0.25In2.75S4 NWs obtained under H2S at 400 °C for 60 min; (c) steady state absorption−transmission spectrum of the Cu2SnS3 and Cu0.25In2.75S4 NWs on fused silica; right inset shows the linear I−Vs obtained from p-type CuS on SLG in the dark and light while the left inset depicts the transmission through 10, 20, and 40 nm CuS with a peak around 600 nm.

which suggests that they consist of Au and In and that the Sn:In2O3 NWs grow by the VLS mechanism. A typical EDX spectrum from the CuS shell alone is also shown in Figure 2c confirming that it consists of Cu and S, while we detected Cu, In, and S from an area near the center, directly over the In2O3 NW. The CuS/Sn:In2O3 NWs obtained at 100 and 200 °C have small resistances of the order of 1−10 Ω due to the high carrier density of electrons in the n-type In2O3 and holes in ptype CuS while they exhibited PL emission at ≈3.5 eV, close to the energy band gap of In2O3.15 In order to understand better the properties of the p-type CuS surrounding the n-type Sn:In2O3, we deposited 10, 20, and C


Article

The Journal of Physical Chemistry C

Figure 4. (a) TEM images of the Cu0.25In2.5S4 NWs obtained at 400 °C; (b) HRTEM of the Cu0.25In2.5S4 NWs showing the (400) and (222) lattice planes also confirmed by SAED shown in (c); (d) EDX spectra obtained from the Cu0.25In2.5S4 NWs confirming the presence of Cu, In, and S.

which undergoes complete decomposition on the surface of oxides, even at room temperature, giving S atoms that bond to the metal cations of the surface. The ionic radii of O2− and S2− are 1.32 and 1.82 Å respectively meaning that S will diffuse into the In2O3 and hence S2− will substitute O2− or fill in vacancies. Hence we observed a suppression of the peaks belonging to the cubic bixbyite crystal structure of In2O3 and the emergence of peaks belonging to Cu0.23In2.59S4 as shown in the XRD of Figure 3b. The steady state transmission spectrum through the p-type Cu0.23In2.59S4 NWs on fused silica is shown in Figure 3c from which it is evident that they absorb in the near-infrared since the energy band gap of the Cu0.23In2.59S4 NWs is 1.5 eV. However, as we shall see later their current carrying capability was smaller than the CuS/Sn:In2O3 NWs as CEs in a QDSSC due to their higher resistance and poor crystallinity at higher processing temperatures. Properties of CuS/SnO2 NWs. The CuS/SnO2 NWs were obtained from SnO2 NWs that were grown at 800 °C on Si(001). The SnO2 NWs typically have diameters of a few tens of nanometers, lengths up to ≈10 μm, and a tetragonal rutile crystal structure with a carrier density of ≈1016 cm−3 and mobility of 70 cm2/(V s) as determined previously from treahertz conductivity spectroscopy.2 The CuS/SnO2 NWs obtained via the deposition of 40 nm Cu over the SnO2 NWs and postgrowth processing under H2S for 60 min at 100−200 °C consist of hexagonal covelite CuS and tetragonal rutile SnO2 and had a dark green color. They exhibited PL emission at ≈550 nm or 2.5 eV related to oxygen vacancies and states lying energetically in the upper half of the energy band gap of SnO2 as described in detail previously but also had small resistances of the order of 10−100 Ω due to the surrounding p-type CuS.15

40 nm Cu on quartz that was converted into CuS under H2S at 100 and 200 °C for 60 min. The CuS had a hexagonal crystal structure and a dark-green color. Cu deficient films such as CuS and Cu1.12S are superior to Cu-rich Cu1.75S and Cu1.8S in their electrocatalytic activity and QDSSCs. In addition, the stability of CuS is substantially better than that of the Cu2S as suggested by Kim et al.22 The transmission coefficient through the 10 nm CuS slide was 0.8 or 80% for λ = 500−800 nm and decreased to 40% upon increasing the thickness to 40 nm as shown by the inset in Figure 3c. More importantly, the resistance of a 40 nm CuS, 15 mm × 5 mm slide with two 100 nm Au contacts deposited on its ends, determined from the linear I−V characteristics, shown as an inset in Figure 3c, was ≈10 Ω or ≈3.3 Ω/□ in the dark and decreased to ≈7 Ω or 2.3 Ω/□ upon exposure to light, while the resistance of the 10 nm CuS in the dark was 50 Ω or 17Ω/□. This low resistance of CuS is attributed to Cu vacancies and the large density of holes which is of the order of 1020 cm−3, not the mobility which is relatively small, i.e., 1−10 cm2/(V s). Now a typical SEM image of the Cu/Sn:In2O3 NWs after postgrowth processing under H2S for 60 min at 300 and 400 °C is also shown in Figure 2a. At 300 °C one may observe the formation of crystals on the surface of the Sn:In2O3 NWs, while at 400 °C we find that the resultant Cu0.23In2.59S4 NWs consist of irregularly stacked crystals with sizes of 60− 70 nm as shown by the TEM image shown in Figure 4a. A HRTEM image of the Cu0.23In2.59S4 crystals is shown in Figure 4b where the lattice spacings are 3.1 and 2.68 Å along the [222] and [400] crystallographic directions, respectively, also confirmed by selective area electron diffraction (SAED) shown in Figure 4c. The formation of Cu0.23In2.59S4 is attributed to the H2S D


Article

The Journal of Physical Chemistry C We find that the Cu2SnS3/SnO2 NWs obtained between 300 and 500 °C had a darker brown color and consisted of tetragonal Cu2SnS3 and SnO2. A TEM image of the Cu2SnS3/ SnO2 NWs is shown in Figure 5a from which it is obvious that

determined from steady state absorption transmission spectroscopy.24 The UV emission from the Cu2SnS3/SnO2 NWs was previously attributed to the electrostatic formation of SnO2 QDs thought to be caused by the irregularity of the p-type Cu2SnS3 shell that could result in a depletion of the SnO2.15 Here we confirm the existence of the SnO2 QDs which have been observed for the first time in nanowires and are responsible for the UV emission via a breakdown of the dipole forbidden rule. Self-Consistent Calculations of Core−Shell p−n Junctions. From the above, one expects the formation of a p−n tunnel junction between the p+-CuS and n+-Sn:In2O3 NWs and an asymmetric p+−n junction between p+-type CuS and n-type SnO2 NWs. The CuS/In2O3 NWs are similar to the Cu2S/Sn:In2O3 core−shell NW p−n tunnel junctions of Jiang et al.17 who decorated Sn:In2O3 NWs with Cu2S quantum dots (QDs) by solution processing methods and used an ITO/ TiO2/CdS/CdSe PA consisting of TiO2 NPs coated with CdS/ CdSe QDs that were deposited on ITO in conjunction with a polysulfide electrolyte. In order to gain a better understanding of the CuS/Sn:In2O3 and CuS/SnO2 core−shell NWs, we have calculated the conduction band (CB) and valence band (VB) potential profile, energetic position of one-dimensional sub-bands, wave function, and one-dimensional electron gas (1DEG) distributions along their radius via the self-consistent solution of the Poisson−Schrödinger (SCPS) equations in cylindrical coordinates and the effective mass approximation as described in detail elsewhere.25 The SCPS calculations were carried out by taking into account (a) me* = 0.35mo and εr = 9.3 for Sn:In2O326,27 (b) me* = 0.2mo and εr = 13.5 for SnO2;28,29 (c) mh* = 0.8mo for CuS30 and εr = 7.0 of CuS.31,32 In addition we have taken into account (a) the work function of CuS, i.e., ϕ ≈ 5.0 eV33 and its electron affinity χ = 1.9 eV;34,35 (b) the CB discontinuity ΔEc ≈ 0.1 eV at the SnO2/CuS and Sn:In2O3/ CuS heterojunctions which is similar to that between ZnO and Cu2S36 but also in accord with F. Säuberlich et al.37 who showed that the large interface dipole moments which exist between ZnO, SnO2, In2O3, and Cu2S result in small ΔEC. The overall band bending and built-in potential barrier height in the case of the CuS/Sn:In2O3 core−shell NWs depends on the difference in energy band gaps and the energetic position of the Fermi level in the n-type Sn:In2O3 and p-type CuS considered separately. The SCPS CB potential profile of the CuS/Sn:In2O3 and CuS/SnO2 core−shell NWs are shown as insets in Figure 6 and Figure 7 respectively. Note that the band bending and potential barrier height of the CuS/Sn:In2O3 heterojunction is larger than that of CuS/SnO2 since the Fermi level resides further below the conduction band edge in SnO2 as a result of the smaller electron density which is of the order of 1016 cm−3.2 In addition one may observe that the SnO2 is depleted in the case of the CuS/SnO2 core−shell NWs, as expected in the case of an asymmetric p+n junction. In contrast the Fermi level resides close to the CB edge in the n-type Sn:In2O3 and close to the valence band of the p-type CuS shell so there are both electrons and holes in the n-type Sn:In2O3 and p-type CuS, and one expects the occurrence of NDR in the case of the CuS/ Sn:In2O3 core−shell NWs. I−Vs of Au, CuS, and Sn:In2O3 Symmetric Cells. However, before considering next the current transport properties of the CuS/Sn:In2O3 and CuS/SnO2 core−shell NW CEs in a QDSSC, it is useful to describe the properties of symmetric cells consisting of (i) Au (ii) CuS and (iii) ITO top

Figure 5. (a) TEM images of Cu2SnS3/SnO2 NWs obtained from Cu/ SnO2 NWs under H2S at 500 °C for 60 min; (b) HRTEM images of the Cu2SnS3/SnO2 NWs showing SnO2 QDs with sizes varying from 2 to 5 nm and the (110), (101) crystallographic planes of SnO2 (c) corresponding SAED pattern of SnO2 QDs and EDX spectrum of Cu2SnS3.

they consist of irregularly stacked crystals along their length similar to the Cu0.23In2.59S4 NWs. A HRTEM image is shown in Figure 5b which shows the (111) and (200) crystallographic planes of Cu2SnS3. Interestingly we also observed the existence of SnO2 QDs with diameters between 2 and 5 nm. The SnO2 QDs have a spherical-like shape and lattice spacings of 3.34 and 2.64 Å along the [110] and [101] crystallographic directions, respectively, in accordance with SAED shown in Figure 5c. The SnO2 QDs in fact appear to exist in the vicinity of the surface as shown by Figure 5a and are responsible for the observation of PL and ultraviolet emission at 340 nm or 3.65 eV. Before elaborating further, we should point out that SnO2 has a direct energy band gap of 3.7 eV, but the even-parity symmetry of the conduction-band minimum and valence-band maximum states prohibits band-edge radiative transitions which has hindered the potential application of SnO2 as an optoelectronic device. However, Li et al.23 realized an electrically driven SnO2 UV LED by thermal annealing of a 500 nm amorphous layer of SnO2 which led to the formation of SnO2 QDs and a suppression of the dipole forbidden rule. Similarly SnO2 QDs with diameters between 5 and 20 nm obtained by solution processing have energy band gaps between 4.1 and 3.8 eV as E


Article


QDSSCs17 but p-type CuS also has a high work function of 5.0 eV and is a low-cost alternative to Au CEs. We find that the I− Vs obtained from a symmetric cell consisting of p-type CuS are linear, as shown by the inset in Figure 8, suggesting the formation of a smaller barrier at the solid−liquid interfaces and ohmic-like behavior. The current carrying capability of the ptype CuS cell is smaller than that of Au due to the difference in their thicknesses on glass, but it has been shown that Cu2−xS in conjunction with ITO/TiO2/CdS/CdSe PAs perform better compared to Au.38,39 However, the main point of focus here is the comparison of the I−Vs between symmetric cells consisting of CuS on one hand and In2O3 on the other. We obtained a symmetric, but nonlinear, I−V from a cell consisting of ITO as shown in Figure 8, which is again related to the formation of a potential barrier at the two solid−liquid interfaces. These differences are important in view of the fact that the liquid electrolyte comes in contact with the p-type CuS but also with the surfaces of the n-type Sn:In2O3 NWs that may not be entirely covered by CuS. I−Vs of CuS/Sn:In2O3 and CuS/SnO2 CEs. Now the I−V characteristics of the CuS/Sn:In2O3 NW CEs obtained from postgrowth processing Cu/Sn:In2O3 core−shell NWs under H2S at 100−200 °C versus plain ITO PAs are shown in Figure 6. These were stable and reproducible after many days, even after taking apart and reassembling the cells. The rectifying, nonsymmetric I−Vs are attributed to the formation of a p−n junction between the p-type CuS and n-type Sn:In2O3 NW CEs even though the CuS shell has an irregular thickness and certain regions of the Sn:In2O3 NWs are expected to come into direct contact with the liquid electrolyte. More specifically the liquid electrolyte comes into contact with the p-type CuS, but the solid−liquid interface behaves like an ohmic contact as shown by the inset in Figure 8. On the other hand, the liquid electrolyte in contact with Sn:In2O3 gives rise to the formation of a barrier and nonlinear I−Vs as shown in Figure 8, so if the surface of the Sn:In2O3 NWs was not covered adequately by CuS we would expect to observe a nonlinear but symmetric I− V. It appears therefore that the nonsymmetric I−Vs of Figure 6 are attributed to the rectifying properties of the core−shell CuS/Sn:In 2 O 3 p−n junction. Now the I−Vs of the Cu0.23In2.59S4 NW CEs obtained at 300−500 °C are not as good as those obtained at lower temperatures. Higher temperatures promote the diffusion of Cu and reaction with In which leads to the formation of irregularly stacked Cu0.23In2.59S4 NWs with apparently higher resistance. Hence the Cu0.23In2.5S4 NWs carry a smaller current compared to the CuS/Sn:In2O3 NW CEs obtained at lower temperatures which are similar to the Cu2S/Sn:In2O3 NWs p−n tunnel junctions of Jiang et al.16 who fabricated Sn:In2O3 NWs decorated with Cu2S QDs by solution processing methods. However, we did not observe any NDR in the I−Vs of the CuS/Sn:In2O3 NW CEs as expected from a tunnel junction. Before elaborating further, we ought to mention that we fabricated a two-terminal device via the deposition of Cu over part of the n-type Sn:In2O3 NWs and converted to CuS/Sn:In2O3 under H2S at 100−200 °C after which we deposited Au over the bare part of the n-type Sn:In2O3 NWs.15 We did not observe NDR in the I−Vs obtained by making contact with the p-type CuS on one side and the n-type Sn:In2O3 NWs on the other, although we got rectifying I−Vs.15 Asymmetric NDR has been observed in twoterminal devices consisting of Cu2S/ZnO core−shell NWs by Liu et al.40 which was attributed to the diffusion of Cu+ within Cu2S. Defect related asymmetric NDR has also been suggested

Figure 6. I−Vs of a cell consisting of CuS/Sn:In2O3 core−shell NW CE and ITO PA as depicted by the schematic diagram shown at the top ; the CuS/Sn:In2O3 core−shell NWs were obtained from Cu/ Sn:In2O3 NWs under H2S at 100 °C, 200 °C while Cu0.25In2.75S4 NWs were obtained at 300 and 400 °C; left top inset shows the SCPS CB and VB edge potential profile relative to the Fermi level, taken at EF = 0 eV, versus distance along the radius of the CuS/Sn:In2O3 NWs; lower right inset shows rectifying I−Vs of a cell consisting of a CuS/ Sn:In2O3 core−shell NW CE and TiO2/ITO PA with different TiO2 thicknesses.

Figure 7. I−Vs of a cell consisting of CuS/SnO2 core−shell NW CE and ITO PA as depicted by the lower right schematic ; the CuS/SnO2 core−shell NWs were obtained from Cu/SnO2 NWs under H2S at 100 °C, 200 °C and Cu2SnS3/SnO2 NWs at 400 and 500 °C; top inset shows the SCPS CB and VB edge potential profile relative to the Fermi level, taken at EF = 0 eV, versus distance along the radius of the CuS/SnO2 NWs.

and bottom electrodes as shown in Figure 8. The I−Vs of a symmetric cell consisting of Au, top and bottom electrodes, are symmetric, but nonlinear, as shown in Figure 8, due to the formation of a potential barrier at the two solid−liquid interfaces, depicted schematically by the energy band diagram, shown as an inset in Figure 8. Gold has a high work function between 5.0 and 5.5 eV and is commonly used as a CE in

Figure 8. I−Vs of a symmetric cell consisting of n-type ITO, top and bottom electrode, but also Au. A schematic diagram of the cell is depicted as an inset at the top; right inset shows the energy band diagram of the n-type ITO in contact with the liquid electrolyte, where a barrier exists at each solid−liquid interface giving rise to two back-toback Schottky like diodes and rectifying like I−Vs; lower inset shows the linear I−Vs obtained from a symmetric cell consisting of p-type CuS. F


Article

The Journal of Physical Chemistry C to occur in GaN NWs by Chu et al.41 although Dragoman et al.42 proposed that the NDR of GaN NWs on interdigitated electrodes on SiO2 is related to tunneling between the GaN NWs. Moreover, NDR has also been observed in ZnO NWs deposited between two metallic electrodes by Zhang et al.43 which was attributed to electron charging and discharging of the parasitic capacitor due to the weak contact. We have observed NDR between n-type Sn:In2O3 NWs grown on Si that were brought into weak contact with p-type CuS on quartz but without the formation of a solid state junction as shown in Figure 9. In a p−n tunnel junction diode, electrons tunnel from

observe any significant open circuit voltage or short circuit current associated with the CuS/Sn:In2O3 and CuS/SnO2 CE NWs. Similarly we did not observe any significant open circuit voltage or short circuit current from the Cu0.23In2.59S4 and Cu2SnS3 NWs although they absorb photons as shown by the steady state transmission spectrum in Figure 3c. The difference in the absorption spectra of Cu0.23In2.59S4 and Cu2SnS3 NWs is attributed to the difference in their energy band gaps. The effect of the TiO2 barrier thickness on the I−V characteristics of a cell consisting of a CuS/Sn:In2O3 CE and TiO2/ITO PA is shown as an inset in Figure 6 from which it is apparent that the reverse current is suppressed. Evidently the thickness of the TiO2 barrier must be carefully controlled in order to maintain transparency and a high current flow, when an absorber layer of TiO2 NPs and CdS/CdSe QDs is deposited on top of the ITO/ TiO2 PA in order to make a QDSSC. Properties of ITO/TiO2 PA. The importance of recombination at the transparent conducting oxide−liquid electrolyte interface has been described by Ruhle et al.20 who showed that a TiO2 layer deposited on the ITO PA can prevent electron hole recombination and increase the overall efficiency. Hence we deposited 5, 10, 20, and 40 nm of Ti on ITO and converted this into TiO2/ITO by thermal oxidation at 500 °C for 60 min, thereby recovering the optical transparency even in the case of the 40 nm thick Ti layer on ITO. We observed weak but nevertheless well resolved XRD peaks, belonging to the anatase crystal structure of TiO2 after thermal oxidation, consistent with previous investigations on the thermal oxidation of Ti carried out between 400 and 1000 °C which resulted into the formation of anatase below 700 °C and a gradual transformation to tetragonal rutile TiO2 between 700 and 800 °C.44,45 The high level of transparency obtained by thermal oxidation at 500 °C is also consistent with the observation of an increase in the optical band gap of TiO2 that occurs between 400 and 600 °C.46 Interestingly we have also observed the occurrence of NDR in connection with the TiO2 barriers that were deposited on the ITO PAs. More specifically we observed (i) symmetric NDR from a symmetric cell consisting of a 10 nm TiO2/ITO, top and bottom electrode, with polysulfide electrolyte in between, as shown in Figure 10, and (ii)

Figure 9. NDR obtained from n-type Sn:In2O3 NWs in contact with p-type CuS on SLG similar to a cat’s whisker device; insets show (i) the energy band diagram of a p−n tunnel junction where electrons tunnel from the CB of the n-type Sn:In2O3 into the VB of the p-type CuS giving rise to an initial increase of current after which (ii) the conduction and valence band states move out of alignment resulting in a reduction of current; (iii) a higher positive bias makes electrons flow from the conduction band of the n-type Sn:In2O3 into conduction band of the p-type Cu2S.

the conduction band on the n+ side into empty holes states on the p+ side so the current will increase immediately under a positive bias as shown in Figure 9, but at some point the empty hole states will move out of alignment with states occupied by electrons resulting into a reduction of the current giving rise to NDR. Larger electric fields will eventually lower the potential barrier, and electrons will flow from the n+ to the p+ side throughout the conduction band. A reverse bias will make electrons in the valence band, on the p-type side, move into empty states in the conduction band on the n-type side resulting into an immediate and large increase of current. Consequently the breakdown voltage is zero in the p−n tunnel diode which exhibits asymmetric NDR. Here we observe NDR with a peak to valley ratio of ≈1.7 at 0.6 V which did not change under light. Clearly the occurrence of NDR in this case is related to the interface between the n-type Sn:In2O3 NWs that were brought into weak contact with p-type CuS, similar to a cat’s whisker device. It appears that the observation of NDR is not be observed in the case of a solid state junction between the p-type CuS and n-type In2O3 due to the fact that the heterojunction is not atomically abrupt. We should note at this point that NDR occurs at 0.6 V which is close to the open circuit voltages of QDSSCs. Now the current asymmetry of the I−Vs obtained from the CuS/Sn:In2O3 NWs is more pronounced than that of the CuS/ SnO2 NW CEs due to the difference in the built-in potential barrier heights as shown by the energy band diagrams in Figure 6 and Figure 7 but also due to the higher resistance of the SnO2 NWs which results in a smaller current carrying capability of the CuS/SnO2 CEs. The I−Vs of both CuS/Sn:In2O3 and CuS/SnO2 CEs combined with transparent ITO as PA did not change upon exposure to light. In other words, we did not

Figure 10. I−Vs and NDR obtained from a symmetric cell consisting of 10 nm TiO2/ITO top and bottom electrode; I−Vs and NDR obtained from a cell consisting of 10 nm TiO2/ITO and 5 nm TiO2/ ITO are shown in the lower right inset. The energy band diagram of the ITO/TiO2/S2−Sn liquid−solid interface in equilibrium is shown at the top depicting the oxygen vacancy states located 0.8 eV below the top of the CB of TiO2.

nonsymmetric NDR from a cell consisting of 40 nm TiO2/ITO bottom and ITO top electrode, as shown in Figure 11. It is also useful to note that the I−Vs of a symmetric cell consisting of 50 nm Ti were linear, suggesting ohmic like behavior in contact with the liquid electrolyte. Now symmetric NDR occurs in double-barrier resonant tunneling diodes or single barrier metal−insulator−metal devices containing defects which may G


Article


Figure 11. I−Vs and NDR obtained from a cell consisting of 40 nm TiO2/ITO top and ITO bottom electrode which is depicted schematically as the top right inset. The I−Vs and NDR are stable and reproducible as shown by overlapping traces in red and blue. The energy band diagram of the ITO/TiO2/S2−Sn liquid−solid interface under the application of an electric field is shown as an inset at the top where the NDR is associated with charging of the oxygen vacancy states situated in the energy gap of TiO2. The increase in current beyond the regime of NDR is related to the lower height of the barrier resulting from the application of a large electric field or bias as shown by the energy band diagram depicted as an inset at the bottom right.

Figure 12. I−V obtained from a QDSSC employing a CuS/Sn:In2O3 NW CE in conjunction with an ITO/TiO2/CdS/CdSe PA depicted schematically as an inset at the bottom left; upper inset shows the energy band diagram of the photoanode and the conversion of photons to electron−hole pairs.

have energy band gaps of ≈2.4 and 1.8 eV respectively and form a type II heterojunction on TiO2 with a conduction band discontinuity of ΔEC ≈ 0.5 eV. When light is incident on the CdS/CdSe QDs, photons are absorbed leading to the generation of electron−hole pairs. The electrons move down hill in energy and are collected by the In2O3, whereas holes receive an electron from the S2− ions which then move toward the CuS/Sn:In2O3 CE and pick up electrons that are delivered again to the CdSe QDs thereby completing the circuit. We have obtained an open circuit voltage of VOC ≈ 0.5 V, short circuit current density of JSC ≈ 2 mA/cm2, and an overall power conversion efficiency of ≈0.4% from a QDSSC consisting of a CuS/Sn:In2O3 CE in conjunction with the ITO/TiO2/CdS/ CdSe PA as shown in Figure 12.

be charged as described by Simmons and Verderber47 and is still an active topic of investigation.48,49 Recently it was shown that highly reproducible, symmetric NDR in TiO2 is related to charge storage and release arising from trapping and detrapping of oxygen ions at defects sites and midgap states distributed over an energy range of 0.65 eV at approximately 0.5−0.8 eV below the conduction band edge.50 The two common defects in TiO2 are oxygen vacancies and Ti interstitial, and the energy band diagram of such a TiO2 tunnel barrier under different applied electric fields is shown in Figure 10 and Figure 11. In this picture, electrons will initially tunnel through the TiO2 barrier leading to an increase in the current, but at some point the states will be charged resulting into a reduction of the current and the observation of NDR. A further increase in electric field will lower the TiO2 barrier even more, and electrons will tunnel through a thinner barrier leading to an increasing current. Water decomposition and absorption have also been shown to play a crucial role in the observation of NDR in oxides such as TiO2 and Al2O3, and here we have observed the occurrence of NDR in connection with TiO2 barriers on ITO using a liquid electrolyte containing S, Na2S in CH3OH and H2O, so attention should be paid to the chemical environment of the oxides when studying the electrical properties of QDSSCs. Interestingly the NDR was not observed in connection with 5 nm TiO2 barrier on ITO which may be due to the smaller density of oxygen vacancies arising from the complete oxidation of the 5 nm Ti layer deposited on ITO. Hence we observe a nonsymmetric NDR from a cell consisting of 10 nm TiO2/ITO CE and 5 nm TiO2 /ITO PA that is shown as an inset in Figure 10. The NDR and I−Vs shown in Figure 10 and Figure 11 are reproducible and stable, consistent with other observations of NDR in TiO2.50 Furthermore, the I−V of the TiO2 barrier is essentially linear between −1 and +1 V, so the NDR related to charging and discharging of defects should not be an impediment to the operation of a QDSSC given that the open circuit voltages are usually smaller than 1 V. Performance of CuS/Sn:In2O3 CEs in QDSSC. The CuS/ In2O3 core−shell NW CEs were combined with an ITO/TiO2/ CdS/CdSe PA. The energy band diagram of the ITO/TiO2/ CdS/CdSe PA in contact with the liquid electrolyte is shown as an inset in Figure 12. The work function of In2O3 is ≈4.8 eV while TiO2 has a band gap of 3.2 eV, work function of 4.2 eV, and electron affinity of χ = 4.0 eV. In addition CdS and CdSe

4. CONCLUSIONS The structural, optical, and electrical transport properties of nanowires obtained by the deposition of Cu over Sn doped In2O3 and SnO2 NWs and processing under H2S between 100 and 500 °C have been investigated for their subsequent use in QDSSCs. We find that the CuS/Sn:In2O3 NWs obtained between 100 and 200 °C consist of hexagonal CuS and cubic In2O3, but higher temperatures lead to the formation of Cu0.23In2.59S4 NWs. Moreover we observed the existence of SnO2 QDs in tetragonal Cu2SnS3 NWs obtained at 400−500 °C which are responsible for UV emission at 3.65 eV and a breakdown of the dipole forbidden rule in SnO2. The CuS/ Sn:In2O3 NWs exhibit rectifying I−Vs with stronger current asymmetry and higher current carrying capability compared to CuS/SnO2 NWs due to the larger built-in potential barrier and higher electron density in the n-type core of the Sn:In2O3 NWs. However, we did not observe NDR which is typical of a p−n tunnel junction in connection with the CuS/Sn:In2O3 NWs probably due to the fact that the heterojunction is not atomically abrupt. NDR was observed by bringing Sn:In2O3 NWs in weak contact with p-type CuS, without the formation of a solid state junction, similar to a cat’s whisker device. In addition, we observed NDR in connection with TiO2 barriers deposited on ITO PAs which is used to prevent electron hole recombination with the liquid electrolyte. The NDR is attributed to charging and discharging of oxygen vacancy traps and states located energetically in the upper half of the energy band gap of TiO2, but this occurs above 1 V so it is probably not an impediment to QDSSCs which have open circuit voltages typically less than 1 V. We obtained a power conversion efficiency of ≈0.4% short circuit current density of JSC ≈ 2 mA/cm2 and open circuit voltage VOC = 0.5 V for the H


Article


(16) Todorov, T. K.; Reuter, K. B.; Mitzi, D. B. An Alternative Route Towards Low-Cost Cu2ZnSnS4 Thin Film Solar Cells. Adv. Mater. 2010, 22, 156−159. (17) Jiang, Y. B.; Zhang, X.; Ge, Q. Q.; Yu, B. B.; Zou, Y. G.; Jiang, W. J.; Song, W. G.; Wan, L. J.; Hu, J. S. ITO/Cu2S Tunnel Junction Nanowire Arrays as Efficient Counter Electrode for Quantum-DotSensitized Solar Cells. Nano Lett. 2014, 14, 365−372. (18) Alibabaei, L.; Farnum, B.; Kalanyan, B.; Brennaman, M.; Losego, M. D.; Parsons, G. N.; Meyer, T. J. Atomic Layer Deposition of TiO2 on Mesoporous Nano ITO: Conductive Core−Shell Photoanodes for Dye-Sensitized Solar Cells. Nano Lett. 2014, 14, 3255−3261. (19) Joanni, E.; Savu, R.; Góes, M.; Bueno, P.; Freitas, J.; Nogueira, A. F.; Longo, E.; Varela, J. A. Dye-Sensitized Solar Cell Architecture Based on Indium-Tin-Oxide Nanowires Coated with Titanium Dioxide. Scr. Mater. 2007, 57, 277−280. (20) Ruhle, S.; Yahav, S.; Greenwald, S.; Zaban, A. Importance of Recombination at the TCO/Electrolyte Interface for High Efficiency Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 17473−17478. (21) Lee, H.; Wang, M.; Chen, P.; Gamelin, D.; Grätzel, M.; Nazeeruddin, Md.K.; Zakeeruddin, S. M. Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process. Nano Lett. 2009, 9, 4221− 4227. (22) Kim, C. S.; Choi, S. H.; Bang, J. H. New Insight into Copper Sulfide Electrocatalysts for Quantum Dot-Sensitized Solar Cells: Composition-Dependent Electrocatalytic Activity and Stability. ACS Appl. Mater. Interfaces 2014, 6, 22078−22087. (23) Li, Y.; Yin, W.; Deng, R.; Chen, R.; Chen, J.; Yan, Q.; Yao, B.; Sun, H.; Wei, S. H.; Wu, T. Realizing a SnO2 Based Ultraviolet LightEmitting Diode via Breaking the Dipole-Forbidden Rule. NPG Asia Mater. 2012, 4, E30. (24) Ahmed, A. S.; Azam, A.; Shafeeq, M. M.; Chaman, M.; Tabassum, S. Temperature Dependent Structural and Optical Properties of Tin Oxide Nanoparticles. J. Phys. Chem. Solids 2012, 73, 943−947. (25) Zervos, M. Properties and Tailoring of the Ubiquitous CoreShell p−n Junction in Semiconductor Nanowires by δ-Doping. Phys. Status Solidi RRL 2013, 7, 894−897. (26) Yi, F.; Shim, E.; Zhu, A. Y.; Zhu, H.; Reed, J. C. Voltage Tuning of Plasmonic Absorbers by Indium Tin Oxide. Appl. Phys. Lett. 2013, 102, 221102. (27) Brewer, S. H.; Franzen, S. Calculation of the Electronic and Optical Properties of Indium Tin Oxide by Density Functional Theory. Chem. Phys. 2004, 300, 285−293. (28) Batzill, M.; Diebold, U. The Surface and Materials Science of Tin Oxide. Prog. Surf. Sci. 2005, 79, 47−154. (29) Button, K.; Fonstand, C. G.; Dreybradt, W. Determination of the Electron Masses in Stannic Oxide by Sub Millimeter Cyclotron Resonance. Rhys. Rev. B: Condens. Matter Mater. Phys. 1971, 4, 4539− 4542. (30) Zhao, Y.; Pan, H.; Lou, Y.; Qiu, X.; Zhu, J.Ji; Burda, C. Plasmonic Cu2‑xS Nanocrystals: Optical and Structural Properties of Copper-Deficient Copper I Sulfides. J. Am. Chem. Soc. 2009, 131, 4253−4261. (31) Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots. Nat. Mater. 2011, 10, 361−366. (32) Güneri, E.; Kariper, A. Optical Properties of Amorphous CuS Thin Films Deposited Chemically at Different pH Values. J. Alloys Compd. 2012, 516, 20−26. (33) Gupta, S.; Batra, Y.; Mehta, B. R.; Satsangi, V. R. Study of Charge Separation and Interface Formation in a Single Nanorod CdS/ Cu x S Heterojunction Solar Cell Using Kelvin Probe Force Microscopy. Nanotechnology 2013, 24, 255703−255708. (34) Nascu, C.; Pop, I.; Ionescu, V.; Indrea, E.; Bratu, I. Spray Pyrolysis Deposition of CuS Thin Films. Mater. Lett. 1997, 32, 73−77.

cell employing CuS/Sn:In2O3 core−shell NWs in conjunction with ITO/TiO2/CdS/CdSe PAs.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Prof. E. Leontidis at the Department of Chemistry for many useful discussions. The SEM analysis was possible due to EU-funding grant POSCCE-A2-O2.2.1-2013-1/Axa Prioritara 2, Project No. 638/12.03.2014, Code SMIS-CSNR 48652.

■

REFERENCES

(1) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nanowire Dye-Sensitized Solar Cells. Nat. Mater. 2005, 4, 455−459. (2) Tsokkou, D.; Othonos, A.; Zervos, M. Carrier Dynamics and Conductivity of SnO2 Nanowires Investigated by THz Conductivity Spectroscopy. Appl. Phys. Lett. 2012, 100, 133101. (3) Tsokou, D.; Zervos, M.; Othonos, A. Ultrafast Spectroscopy of In2O3 Nanowires Grown by the Vapor−Liquid−Solid Method. J. Appl. Phys. 2009, 106, 084307. (4) Zervos, M.; Mihailescu, C.; Giapintzakis, J.; Luculescu, C.; Florini, N.; Komninou, P.; Kioseoglou, J.; Othonos, A. Broad Compositional Tunability of Indium Tin Oxide Nanowires Grown by the Vapor−Liquid−Solid Mechanism. APL Mater. 2014, 2, 056104. (5) Tang, J. Y.; Huo, Z. Y.; Brittman, S.; Gao, H.; Yang, P. D. Solution Processed Core Shell Nanowires for Efficient Photovoltaic Cells. Nat. Nanotechnol. 2011, 6, 568−572. (6) Gupta, S.; Batra, Y.; Mehta, B. R.; Satsangi, V. R. Study of Charge Separation and Interface Formation in a Single Nanorod CdS−CuxS Heterojunction Solar Cell Using Kelvin Probe Force Microscopy. Nanotechnology 2013, 24, 255703. (7) Siol, S.; Strater, H.; Bruggemann, R.; Brötz, J.; Bauer, G. H.; Klein, A.; Jaegermann, W. PVD of Copper Sulfide Cu2S for Pin Structured Solar Cells. J. Phys. D: Appl. Phys. 2013, 49, 495112. (8) Liu, Z.; Han, J.; Han, L.; Guo, K.; Li, Y.; Cui, T.; Wang, B.; Liang, X. Fabrication of ZnO/CuS Core/Shell Nano Arrays for Inorganic− Organic Heterojunction Solar Cells. Mater. Chem. Phys. 2013, 141, 804−809. (9) Liu, X.; Mayer, M.; Wang, D. Negative Differential Resistance and Resistive Switching Behaviors in Cu2S Nanowire Devices. Appl. Phys. Lett. 2010, 96, 223103. (10) Jiang, X.; Xie, Y.; Lu, J.; He, W.; Zhu, L.; Qian, Y. Preparation and Phase Transformation of Nanocrystalline Copper Sulfides (Cu9S8, Cu7S4, and CuS) at Low Temperature. J. Mater. Chem. 2000, 10, 2193−2196. (11) Sangamesha, M.; Pushpalatha, K.; Shekar, G.; Shamsundar, S. Preparation and Characterization of Nanocrystalline CuS Thin Films for Dye-Sensitized Solar Cells. ISRN Nanomater. 2013, 2013, 1−8. (12) Thanikaikarasan, S.; Mahalingam, T.; Kathalingam, A.; Moon, H.; Kim, Y. D. Characterization of Electrodeposited Copper Sulphide Thin Films. J. New Mater. Electrochem. Syst. 2010, 13, 29−33. (13) Wu, C.; Zhang, Z.; Wu, Y.; Lv, P.; Nie, B.; Luo, L.; Wang, L.; Hu, J.; Jie, J. Flexible CuS Nanotubes−ITO Film Schottky Junction Solar Cells with Enhanced Light Harvesting by Using an Ag Mirror. Nanotechnology 2013, 24, 045402. (14) Zervos, M.; Mihailescu, C.; Othonos, A.; Giapintzakis, J.; Luculescu, C. Sulphur Passivation and the Conversion of SnO2 to SnS2 Nanowires. Mater. Sci. Eng., B 2015, 198, 10−13. (15) Karageorgou, E.; Zervos, M.; Othonos, A. Ultraviolet Emission from Low Resistance Cu2SnS3/SnO2 and CuInS2/Sn:In2O3 Nanowires. APL Mater. 2014, 2, 116107. I


Article

The Journal of Physical Chemistry C (35) Panigrahi, S.; Basak, D. Solution-Processed Novel Core−Shell n−p Heterojunction and Its Ultrafast UV Photodetection Properties. RSC Adv. 2012, 2, 11963−11968. (36) Siol, S.; Strater, H.; Bruggemann, R.; Brotz, J.; Bauer, G. H.; Klein, A.; Jaegermann, W. PVD of Cu2S for Pin-Structured Solar Cells. J. Phys. D: Appl. Phys. 2013, 46, 495112. (37) Säuberlich, F.; Klein, A. Band Alignment at Oxide Semiconductor Interfaces. MRS Online Proc. Libr. 2013, 763, 763−766. (38) Giménez, S.; Seró, I. M.; Macor, L.; Guijarro, N.; Villarreal, T. L.; Gómez, R.; Diguna, L. J.; Shen, Q.; Toyoda, T.; Bisquert, J. Improving the Performance of Colloidal Quantum-Dot-Sensitized Solar Cells. Nanotechnology 2009, 20, 295204. (39) Radich, J. G.; Dwyer, R.; Kamat, P. V. Cu2S Reduced Graphene Oxide Composite for High-Efficiency Quantum Dot Solar Cells. Overcoming the Redox Limitations of S2−/Sn 2‑ at the Counter Electrode. J. Phys. Chem. Lett. 2011, 2, 2453−2460. (40) Liu, Z.; Mayer, M. T.; Wang, D. Negative Differential Resistance and Resistive Switching Behaviors in Cu2S Nanowire Devices. Appl. Phys. Lett. 2010, 96, 223103. (41) Chu, W.; Chiang, H.; Liu, C.; Lai, Y.; Hsu, K.; Chung, H. Defect Induced Negative Differential Resistance of GaN Nanowires Measured by Conductive Atomic Force Microscopy. Appl. Phys. Lett. 2009, 94, 182101. (42) Dragoman, M.; Konstantinidis, G.; Cismaru, A.; Vasilache, D.; Dinescu, A.; Dragoman, D.; Neculoiu, D.; Buiculescu, R.; Deligeorgis, G.; Vajpeyi, A. P.; Georgakilas, A. Negative Differential Resistance in GaN Nanowire Network. Appl. Phys. Lett. 2010, 96, 053116. (43) Zhang, Y.; Lee, C. T. Negative Differential Resistance in ZnO Nanowires Bridging Two Metallic Electrodes. Nanoscale Res. Lett. 2010, 5, 1492−1495. (44) Zhou, B.; Jiang, X.; Liu, Z.; Shen, R.; Rogachev, A. V. Preparation and Characterization of TiO2 Thin Film by Thermal Oxidation of Sputtered Ti Film. Mater. Sci. Semicond. Process. 2013, 16, 513−519. (45) Chowdhury, P.; Barshilia, H. C.; Selvakumar, N.; Deepthi, B.; Rajam, K. S.; Chaudhuri, A. R.; Krupanidhi, S. B. The Structural and Electrical Properties of TiO2 Thin Films Prepared by Thermal Oxidation. Phys. B 2008, 403, 3718−3723. (46) Sartale, S. D.; Ansari, A. A.; Rezvani, S. J. Influence of Ti Film Thickness and Oxidation Temperature on TiO2 Thin Film Formation via Thermal Oxidation of Sputtered Ti Film. Mater. Sci. Semicond. Process. 2013, 16, 2005−2012. (47) Simmons, L.; Verderber, R. New Conduction and Reversible Memory Phenomena in Thin Insulating Films. Proc. R. Soc. London, Ser. A 1967, 301, 77−102. (48) Bozano, L.; Kean, B.; Deline, R.; Salem, J.; Scott, J. Mechanism for Bi Stability in Organic Memory Elements. Appl. Phys. Lett. 2004, 84, 607−609. (49) Du, Y.; Pan, H.; Wang, S.; Wu, T.; Feng, Y. P.; Pan, J.; Wee, A. T. S. Symmetrical Negative Differential Resistance Behavior of a Resistive Switching Device. ACS Nano 2012, 6, 2517−2523. (50) Li, J.; Zhang, H. F.; Shao, G. Q.; Wu, B. L.; Ouyang, S. X. Negative Differential Resistance: Another Banana. Europhys. Lett. 2014, 108, 27005−27015.

J


SnO2

Recommend Documents