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Growth of large scale, large size, few layered #MoO3 on SiO2 and its photoresponse mechanism Yu Wang, Xiang Du, Jiming Wang, Mingze Su, Xi Wan, Hui Meng, Weiguang Xie, Jian-Bin Xu, and Pengyi Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13743 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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ACS Applied Materials & Interfaces

Growth of large scale, large size, few layered α-MoO3 on SiO2 and its photoresponse mechanism Yu Wang,b Xiang Du,b Jiming Wang,b Mingze Su,b Xi Wan,c,d Hui Meng,b Weiguang Xiea,b,e* Jianbin Xu, c* and Pengyi Liu b

a

Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Jinan University, Guangzhou, Guangdong, 510632, P. R. China

b

Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Department of Physics, Jinan University, Guangzhou, Guangdong, 510632, P. R. China

c

Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Hong Kong SAR, P. R. China

d

Engineering Research Center of IoT Technology Applications (Ministry of Education), Department of Electronic Engineering, Jiangnan University, Wuxi, Jiangsu, 214122, P. R. China

e

State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou, Guangdong, 510275, P. R. China

KEYWORDS: MoO3; two dimensional materials; physical vapor deposition; surface defects; photoresponse

*

Author to whom correspondence should be addressed: WG Xie: [email protected]; JB Xu: [email protected]; 1

ACS Paragon Plus Environment


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ABSTRACT Layered α-MoO3 is a multifunctional material that has significant application in opto-electronic devices. In this study, we show the growth of large scale, large size few layered (FL) α-MoO3 nanosheet directly on technical substrates (SiO2 and Si) by physical vapor deposition. We suggest that the growth is self-limiting on [010] direction because of the re-evaporation and high diffusion capacity of MoOx species at high temperature. As prepared FL α-MoO3 is non-conductive and shows poor response to photo illumination with wavelength of 405 nm and 630 nm. Its work function is strongly altered by the substrate. Improvement of conductivity and photoresponse is observed after the FL device is annealed in vacuum. Line defects along [001], [100] and [101] directions belonging to the generation of Os and Oa vacancy states appear, and the interfacial effect is suppressed. Scanning near field optical microscope shows that the defects are absorption sites. Kelvin probe force microscope reveals decrease of apparent work function under illumination, which confirms that electrons are excited from defects states. Our findings show that intense studies on defect engineering are required to push forward the application of two dimensional metal oxides.

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1. INTRODUCTION The intriguing properties of graphene-like materials have led to a wave of research upsurge on design, growth of two dimensional (2D) materials, and investigation on their properties and fabrication of devices. Functional oxides are stable and low-cost. They have diverse optoelectrical properties for various applications. Functional oxides with two dimensional phase normally possess similar properties, while shrinking to two dimensional layer has been found to give superior performance in dielectric properties,1 energy storage,2-3 and optoelectrical response.4-5 MoO3 is one of the multifunctional materials with high work function (~6.9 eV) and wide band gap (~ 3 eV).6-7 There are three main crystalline polymorphs: orthorhombic MoO3 (α-MoO3), monoclinic MoO3 (β-MoO3), and hexagonal MoO3 (h-MoO3). α-MoO3 is a thermodynamically stable phase with layered structures (Figure 1). The double-layers of linked distorted MoO6 octahedra share corners in the directions of [100] and [001]. The layers stake up through weak van der Waals forces along [010] direction. S. Balendhran et al. have shown that when the thickness of α-MoO3 was reduced to 11 nm by mechanical exfoliation, it still had a high relative dielectric constant of 45. It was helpful to suppress the Column scattering effect of electrons, thus high field effect mobility of 1100 cm2/Vs could be obtained in reduced MoO3-x.5 However, the nanoplate prepared by mechanical exfoliation is small and the method is inefficient. By using solution method, few layered (FL) α-MoO3 with thickness down to several nanometers can be prepared more effectively.8-9 M. B. Sreedhara et al. showed that the photocatalytic activity of FL α-MoO3 was better than the bulk sample.8 Manal M. Y. A. Alsaif et al. showed that by tuning the non-stoichiometric ratio of α-MoO3-x, on/off ratio >105 could be realized in field effect transistor.9 Plasmon resonance was also found in 2D sub-stoichiometric α-MoO3, and the wavelength could be tuned by the size of the nanosheet.10-11 However, the typical length scale of a single nanosheet is normally smaller than 1 µm. Recently, D. Wang et al. demonstrated van der Waals epitaxial method using mica as a substrate. They prepared α-MoO3 sheet with thickness of single-unit-cell and length scale of tens of micrometers.12 A. J. Molina further developed this method and grew FL MoO3 up to centimeter scale on mica.13 However, mica is not a normally used substrate for optoelectrical applications. A. J. Molina et al. transferred the FL MoO3 to silicon oxide surface and investigate its optoelectrical response.13 However, transfer of MoO3 is very easy to induced stress, defects or organic residues in the sample, which would significantly alter the properties FL sample, as has been revealed in transferred graphene.14 Therefore, direct growth of large scale, large size FL MoO3 on technical substrate such as Si and SiO2 is still required. 3



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Intrinsic MoO3 is an indirect wide band gap semiconductor. The superior optoelectrical properties depend on post treatment such as hydrogenation and annealing. The mechanism of the treatment has been studied for decades. Experimentally, gap states lying between the Fermi level and the valance states were observed once the MoO3 was hydrogenated or annealed in vacuum. Generation of oxygen defects has been proposed based on the results of Raman spectroscopy, X-ray photoelectron spectroscopy.15-18 Theoretical calculation shows that the oxygen sites can be distorted or even released, forming oxygen vacancy.19-20 The formation of oxygen vacancy sites leads to distortion of neighboring Mo atoms, forming a defect band in the band gap.19, 21 The experimental results and the theoretical prediction consist roughly with each other. But if further investigate on the structure of MoO3 in Figure 1, there are three types of oxygen sites: the terminal oxygen Ot, the doubly coordinated and asymmetric bridging oxygen Oa, and the triply coordinated and symmetric bridging oxygen Os. Recently, K. Inzani et al. showed that the defect bands from Vot, Voa were localized, while the defect band from Vos was more disperse.21 Besides, the formation energies of the defects were different.20-23 With shrinking thickness, details understanding on defects, such as the types, the generation, the stability, and how they work on the optoelectrical properties would become even significant. However, the studies in nanoscale are still in its infancy. The FL α-MoO3 shows atomic flat surface that is an ideal platform to study the defect effect. W. G. Xie et al. has observed the generation of nanostructure during the hydrogenation process of α-MoO3 using scanning probe microscope. The relationship between the nanostructure and the hydrogen sensing properties can be established by in-situ investigation.24 It demonstrates the possibility to reveal the defect properties in nanoscale. In this study, we show that using thermal physical vapor deposition method, large scale FL α-MoO3 can be directly grown on technical substrates such as Si and SiO2. Two electrodes device is fabricated and its photoresponse is investigated. By using Kelvin probe microscope and scanning near field optical microscope, correlation among the materials structure, opto-electrical distribution and device performance are established. We show that line defects are the origin of the change of optoelectric properties, and the mechanism is proposed.

2. EXPERIMENTAL METHODS 2.1 Growth of MoO3 on technical substrate The MoO3 nanosheets were synthesized by thermal physical deposition method under air condition. The silicon and silicon oxide substrate were cleaned by sonication in acetone, alcohol, and distilled water 4


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successively. Each cleaning step lasted for 10 minutes. The substrates removed from the distilled water were dried with nitrogen gas. MoO3 powder of 0.1 g was placed in an Al2O3 crucible at the center of a quartz tube. The substrates were placed at the low temperature region. The distance between the substrate and powder is 13-14.5 cm. The temperature of the source is increased to 780 ℃ from room temperature in 70 minutes, maintaining for 120 minutes and then cooling down to room temperature naturally. The temperature of different positions of sample was measured by a thermal couple. 2.2 Materials characterization The crystal structure of nanosheets is characterized by X-ray diffraction (XRD) spectroscopy in a Rigaku-MiniFlex600 using the Cu κα radiation (1.546 Å), with 40 kV and current of 40 mA. The atomic structure was characterized by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) experiments were conducted on JEOL 2100F operated at 200 kV. The morphologies of the samples were characterized by scanning electron microscopic images taken by ZEISS ULTRA 55 FESEM. Raman spectroscopy measurements were carried out using a confocal Raman microscope (Renishaw inVia Reflex) with a 532 nm laser as excitation. The XPS measurements were performed on a Thermo VG ESCALAB 250 X-ray photoelectron spectrometer using a monochromated Al Kα (1486.6 eV) X-ray source. The light spot of X-ray is around 500 µm in diameter, which is much large than one nanoplate. The spectra is thus a average effect of many nanoplates. Scanning near field optical microscope (SNOM) was conducted using a scattering-type near-field optical microscope (NeaSNOM, Neaspec GmbH). The AFM tip coated with metal layer was illuminated using a visible laser of 633 nm. The backscattered light from the tip was detected via a pseudoheterodyne interferometric manner. The optical and morphology images of the sample can be simultaneously obtained by scanning the sample below the tip.24

2.3 Devices fabrication & measurement Thick MoO3 plate was used as a shadow mask. It was moved by a tungsten probe to the center surface of the MoO3 nanosheet, with both ends uncovered. The sample was then treated by oxygen plasma, and then Au film with thickness of ~ 100 nm was deposited under vacuum of 1×10-3 Pa. The thick MoO3 was then removed and the Au film was scratched apart as two electrodes. Photoresponse was measured in a home-made vacuum chamber. The current-voltage (I-V) curves were taken using Keithley 2612 source meter. The surface potential of the device was measured by Kelvin probe force microscopy in DI multimode Ⅵ. During the measurement, the bias is applied to the tip. The gold electrode is connected to earth. The surface potential of gold electrode is used as reference. 5



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3. RESULTS AND DISCUSSION 3.1 Growth of FL-MoO3 Figure 2(a) shows the deposition process of the MoO3. The deposited morphology of MoO3 is sensitive to the growth conditions. (Figure S1(a)-(c)). When the source temperature is raised to 780 oC, clear morphology difference was found on the substrate (Figure 2(b)). At substrate temperature lower than ~ 580 o

C, we found that randomly orientated micro plate grew on the substrate surface. This morphology has

been observed in many former studies.25-27 As the substrate temperature increases, we found that more and more MoO3 lay down. The size gets larger, and the thickness gets smaller at the same time (Figure S1(d)-(e)). The upward orientated nanoplates can be removed by purge of nitrogen gun or an adhesive tape. The XRD pattern in Figure 2(c) shows the strongest diffraction peaks for the lying down nanosheets lie at 12.8°, 25.7°, 39°, which correspond to (020), (040), (060) planes of α-MoO3 (JCPDS: 05-0508), respectively. In addition, The TEM image and the selected area electron diffraction (SAED) pattern in Figure 2(d) shows that the as-grown crystal has lattice spacing of 3.75 Å and 3.91 Å, which are consistent with the inter-plane distances of (001) and (100) plane of α-MoO3. The color of the nanosheet depends on the thickness and substrate. A red nanosheet in Figure 3(a) shows thickness of 89.8 nm. A yellow nanosheet in Figure 3(b) shows thickness of 40.7 nm. The thinnest nanosheet we found on SiO2 is light green, normally has average thickness of 8 nm. Figure 3(c) shows a nanosheet with thickness of 5.3 nm, it constants ~ 7 MoO3 layers. It’s about 24.2 µm in width and 69.6 µm in length. The MoO3 nanosheet can also grow on silicon surface while the as grown nanosheet has light red color when the thickness is around 10 nm (Figure S2). Raman spectra in Figure 3(d) on the thick nanosheet shows typical peaks of MoO3 at 159 cm-1 (translation vibration of the rigid chains along c axis), 819 cm-1 (stretching vibration of the Mo-O-Mo along a axis) and 993 cm-1 (stretching vibration of Mo-O1 bonds along b axis). As the thickness decreases, the peak intensity decreases. The nanosheet with thickness smaller than 10 nm shows small peaks at 819 cm-1 and 159 cm-1.12 The growth of FL MoO3 can be understood by considering the nucleation and diffusion of MoOx species during the catalyst free vapor solid process.28-29 The growth rate of different facets of MoO3 has been systematically investigated by H C Zeng et al..29 They found that the (010) facet shows the highest growth rate and thus the MoO3 always shows the plate-like structure with the largest facet of (010). However, with increasing supersaturation, a large set of high index facet can be grown.29 In the physical vapor deposition in Figure 2(a), the evaporated MoOx vapor forms a saturation region in the middle of the 6


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tube. Once the temperature decreases, supersaturation appears, leading to nucleation of MoOx. The growth of MoO3 on the substrate depends on the nucleation and diffusion of MoOx species. At moderate temperature region, the energy of the MoOx is lower so that it’s not easy to move on the substrate. High index facets with higher surface energy can also nucleate and grow. This induced the random growth of MoO3 nanoplate on the substrate. At temperature higher than ~ 580 oC, the energy of MoOx is high enough for the MoOx to diffuse freely on the substrate, or even re-evaporated from the substrate (Figure S1 (g)-(k)). This first ensures the nucleation of the most stable (010) facet parallel on the substrate. In addition, the free MoOx species can move to the step edge of the nucleated MoO3 to promote the growth of the nanoplate. Those cannot move to the edge can be re-evaporated, which induces a self-limiting growth of MoO3 along the [010] direction. The growth of ultrathin α-MoO3 sheets is observed at much lower temperature range of 350-480 oC on Mica surface by D. Wang et al..12 This should be due to the lower surface energy and smoother surface of Mica, which facilitates longer diffusion length of MoOx. 3.2 Photoresponse of FL-MoO3 The photoresponse of the FL MoO3 nanosheet was investigated by a two probe method (Figure 4(a)). Most of the as prepared FL devices show high resistivity (1010 Ω or smaller at 1 V, Figure 4(b)) and no response to optical excitation. To induce photoresponse, the devices are exposed to plasma treatment or vacuum annealing. Both treatments cause decrease of device resistivity and significant increase of photocurrent, but the latter gives better long term stability. Figure 4(c) shows the photoresponse of near band gap excitation with wavelength of 405 nm (3.06 eV). The photocurrent response time (τ) was estimated using an exponential function, R=R0 exp(-t/τ). It gives the increase time of 34 s and decay time of 21 s respectively at 90 mW illumination. The photocurrent increases to 33.8 nA, correspond to responsivity of 270 mA/W. It is also found that the device was also responsive to excitation energy (1.94eV, 638 nm) smaller than band gap energy. The photocurrent is 12.5 nA at 90 mW illumination, corresponding to the responsivity of 140 mA/W. It’s found that the photoresponse is stable (Figure S3(a)), and that the response in oxygen is similar but smaller (Figure S3(b)), which means that the response current is not caused by photochromic effect. It’s found that the dark current depends on the treatment process. Longer treatment time causes larger dark current. More importantly, the larger the dark current is, the higher response the devices shows. Figure S4 shows that on the device with smaller dark current, the corresponding responsivity decrease to a smaller value of 1.9 mA/W and 1.8 mA/W at 90 mW illuminations. This means that responsivity and uniformity depends on the dark current. 3.3 Defects effect on photoresponse 7



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Normally, the increase of dark current can be attributed to the generation of gap states. In previous experiments, hydrogen treatment has been applied to tune the opto-electrical properties.5,

17, 30

The

intercalation of hydrogen is able to cause HxMoO3, which is metallic. At high hydrogen concentration, the hydrogen atom can bonded to the lattices oxygen atoms to form OH2 group, whose desorption causes oxygen vacancy defects. A gap states below the Fermi level is observed at this stage.9 Both the HxMoO3 and oxygen vacancy cause change of optoelectrical properties, but are different in nanoscale.

24

In our

experiment, we choose vacuum annealing so as to avoid the interference from hydrogen. As a result, we can focus on structural defects effect. Figure 4(a) shows that on the surface of MoO3 before annealing, no structure defects are observed. On the surface annealed in vacuum in Figure 5(a) & (b), obvious defects along the [100], [001] and [101] direction were observed. The line defects present as small protrusions or cracks. The change of defect morphology comes from successive process of oxygen vacancy generation. The oxygen will first distort to generate a protrusion and then crack when desorption occurred.24 We have observed more than 10 devices and find that the defects randomly appear on devices along the above three directions (Figure S5).Under this condition, the materials still kept its 2D structure. Longer treatment will finally cause the amorphization of the FL nanosheets (Figure S5(c)). It’s been reported that the work function of the MoO3 is 5.2-5.4 eV in air, higher than the work function of Au (5.1 eV). 31-32 However, we observed that the work function of Au is higher than MoO3 (Figure 5(c) & (d)). It’s been reported that the work function of 2D layer on the surface is strongly affected by the substrate materials.33-34 The surface of SiO2 normally has negative charge from Si-O or Si-OH.35 The interfacial dipole can induce positive charge (hole trapped) in the as-prepared, non-conductive MoO3, which leads to the lower apparent work function. Three types of oxygen vacancies from the Ot, Os and Oa sites can be formed theoretically.21-22 From our experiments, the generation of line defects along [001] and [100] indicates the generation of Oa and Os line oxygen vacancies respectively, and the [101] defect is composed both defects (Figure 1). As the formation energy of Ot is smaller than that of Os and Oa,21 it can be expected that Ot also generated. Because of the release of oxygen atoms by vacuum annealing, the neighboring MoO3-x octahedra distorted, forming MoO3-x clusters. The reduction of Mo atom is supported by the XPS results (Figure 6(a)). The as-prepared MoO3 nanoplate shows two strong Mo peaks at 232.76 eV (3d5) and 235.88 eV (3d3), corresponding to the +6 oxidation states. Small peaks at 231.75 eV and 235.05 eV related to the +5 states can be detected due to the high density of edge defects around the nanoplates. After annealing, the peaks of +5 states shift to 231.43 8


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eV and 234.87 eV respectively, and the intensity triples. It indicates the increase of +5 states, and thus the generation of oxygen defects in the annealed sample, which is consistent with the AFM results. The Mo atoms can overlap by the 4d orbitals. The d-d defect states from Ot, Oa and Os lie in the band gap21, which overlap as a defect bands between the Fermi level and valance band.18, 21 Generation of defects band is also confirmed in the valance structure of the annealed sample (Figure 6(b)). The valance edge shift from 2.94 eV to 2.31 eV, and a small band centered at 1.78 eV appears. Raman spectroscopy measurement also shows obvious decrease of the peak at 819 cm-1, which indicates that the Mo-O-Mo bond is broken after annealing (Figure S6). The defects states will generate electron carrier, leading to increase of dark conductivity. The higher the defect density is, the higher the dark current is. The increase of conductivity from defects will also partially screen the interfacial dipole, which explains the increase of work function after annealing in Figure 5(d). The photoresponse of as-prepared FL MoO3 is not detectable because the MoO3 is an indirect band gap semiconductor. Photoresponse of the annealed sample is then related to the oxygen vacancy states. Recent theoretical studies show that O2 will dissociate and annihilate the surface vacancies once MoO3 is exposed to gaseous oxygen.22 Figure S3(b) shows that in oxygen ambient, the device also shows obvious photoresponse, but it’s weaker than in vacuum. The device in air shows the same behavior. This experiment supports that oxygen vacancy states are responsible to the photoresponse. We further performed a local optical absorption measurement on single defects using SNOM. Figure 7 shows that on a line defect, the O4 intensity signal is smaller than the surrounding area. This indicates that the absorption to the light with wavelength of 633 nm is stronger on the defect site. Therefore, we confirm that the defects are the nanoscale origin of photoresponse effect. We further explore the non-equilibrium transport mechanism under illumination using KPFM. Figure 8(a) shows that under illumination, the average surface potential increases, and thus the relative average work function decreases from -72.4 meV to -138.6 meV (Figure 8(b)). This means that the Fermi level shifts downward compared to the Fermi level of gold, implying that the FL MoO3 becomes positive charged. Therefore, a picture of photoresponse can be illustrated in Figure 8(c). Electrons in gap states are excited to the conduction band to generate current flow. Light with wavelength of 638 nm can only excite the electrons in the gap states with energy level down to 1.94 eV below the conduction band minimum, while the light with wavelength of 405 nm can excite the whole defect bands. Because of the extraction of electron from the defect states, the defect states become positively charged. The charged defects are immobile, which lead to a reduction of the Fermi level and thus decrease of apparent work function. 9



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Our findings clearly show the correlation between the oxygen vacancy defects and the optoelectrical conversion in 2D MoO3. For 2D transition metal oxide (TMO), there are high densities of oxygen sites. The generation of oxygen vacancy should be able to change the electrical, optical, mechanical and magnetic properties,36 which provide a feasible way to adjust properties of 2D TMO in a wide range. In addition, different sites of oxygen may provide oxygen vacancy with different types of properties. Local controlling of these defects may be attractive in building nanodevices. It’s thus considered that defect engineering may be one of the important way that required intense studies so as to push forward the application of 2D TMO.

4. CONCLUSIONS We have grown large scale, FL α-MoO3 on technical substrate with typical length scale of tens of micrometers by physical vapour deposition. Increasing the substrate temperature will ensure the nucleation and growth of the most stable (010) facet parallel to the substrate and limited grown along [010] direction. Oxygen line defects generated along [100], [001] and [101] direction determine the opto-electrical properties of FL MoO3. They increase the conductivity as well as the photoresponse. Surface potential measurement shows that the generation of defects is able to screen the substrate effect. The defects states under illumination become positively charged, which confirm that electrons are excited from defects states. Our findings show that oxygen vacancy defects in the 2D MoO3 play key role in opto-electrical conversion, which are likely to hold true in other 2D TMO materials.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grants Nos. 11574119, 61674070, 11474364, 61229401), the Guangdong Natural Science Foundation (Grants Nos. 2014A030313381), and the Research Grants Council of Hong Kong (Grant Nos. AoE/P-03/08, N_CUHK405/12, T23-407/13-N, AoE/P-02/12, CUHK1/CRF/12G, 14207515) and the CUHK Group Research Scheme, as well as Innovation and Technology Commission (Grant No. ITS/096/14).

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website: growth of MoO3 10


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nanoplate on SiO2; growth of MoO3 nanoplate on Si; stability and ambient test; photoresponse of MoO3 FL device with smaller dark current; AFM images of different samples after annealing; Raman spectroscopy before and after annealing.

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(11) Cheng, H.; Qian, X.; Kuwahara, Y.; Mori, K.; Yamashita, H., A Plasmonic Molybdenum Oxide Hybrid with Reversible Tunability for Visible-Light-Enhanced Catalytic Reactions. Adv. Mater. 2015, 27, 4616-4621. (12) Wang, D.; Li, J.-N.; Zhou, Y.; Xu, D.-H.; Xiong, X.; Peng, R.-W.; Wang, M., Van Der Waals Epitaxy of Ultrathin α-MoO3 Sheets on Mica Substrate with Single-Unit-Cell Thickness. Appl. Phys. Lett. 2016, 108, 053107. (13) Molina-Mendoza, A. J.; Lado, J. L.; Island, J. O.; Niño, M. A.; Aballe, L.; Foerster, M.; Bruno, F. Y.; López-Moreno, A.; Vaquero-Garzon, L.; van der Zant, H. S. J.; Rubio-Bollinger, G.; Agraït, N.; Pérez, E. M.; Fernández-Rossier, J.; Castellanos-Gomez, A., Centimeter-Scale Synthesis of Ultrathin Layered MoO3 by Van Der Waals Epitaxy. Chem. Mater. 2016, 28, 4042-4051. (14) Xie, W.-G.; Lai, X.; Wang, X.-M.; Wan, X.; Yan, M.-L.; Mai, W.-J.; Liu, P.-Y.; Chen, J.; Xu, J.-b., Influence of Annealing on Raman Spectrum of Graphene in Different Gaseous Environments. Spectrosc. Lett. 2014, 47, 465-470. (15) Ou, J. Z.; Campbell, J. L.; Yao, D.; Wlodarski, W.; Kalantar-zadeh, K., In Situ Raman Spectroscopy of H2 Gas Interaction with Layered MoO3. J. Phys. Chem. C 2011, 115, 10757-10763. (16) Yan, B.; Zheng, Z.; Zhang, J.; Gong, H.; Shen, Z.; Huang, W.; Yu, T., Orientation Controllable Growth of Moo3 Nanoflakes: Micro-Raman, Field Emission, and Birefringence Properties. J. Phys. Chem. C 2009, 113, 20259-20263. (17) Vasilopoulou, M.; Douvas, A. M.; Georgiadou, D. G.; Palilis, L. C.; Kennou, S.; Sygellou, L.; Soultati, A.; Kostis, I.; Papadimitropoulos, G.; Davazoglou, D.; Argitis, P., The Influence of Hydrogenation and Oxygen Vacancies on Molybdenum Oxides Work Function and Gap States for Application in Organic Optoelectronics. JACS 2012, 134, 16178-16187. (18) Liang, Z.; Su, M.; Zhou, Y.; Gong, L.; Zhao, C.; Chen, K.; Xie, F.; Zhang, W.; Chen, J.; Liu, P.; Xie, W., Interaction at the Silicon/Transition Metal Oxide Heterojunction Interface and Its Effect on the Photovoltaic Performance. Phys. Chem. Chem. Phys. 2015, 17, 27409-27413. (19) Huang, P.-R.; He, Y.; Cao, C.; Lu, Z.-H., Impact of Lattice Distortion and Electron Doping on α-MoO3 Electronic Structure. Sci. Rep. 2014, 4, 7131. (20) Lei, Y.-H.; Chen, Z.-X., Dft+U Study of Properties of MoO3 and Hydrogen Adsorption on MoO3(010). J. Phys. Chem. C 2012, 116, 25757-25764. (21) Inzani, K.; Grande, T.; Vullum-Bruer, F.; Selbach, S. M., A Van Der Waals Density Functional Study of MoO3 and Its Oxygen Vacancies. J. Phys. Chem. C 2016, 120, 8959-8968. (22) Lei, Y.-H.; Chen, Z.-X., A Theoretical Study of Stability and Vacancy Replenishing of MoO3(010) 12


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Figure Captions Figure 1 Schematic structure of α-MoO3: the thickness of one MoO3 layer is 0.7 nm; there are three oxygen sites: Ot, Oa and Os; the breaking of these sites can form three types of line defects Figure 2 (a) Schematic set-up of the growth process of MoO3 in physical vapor deposition system; (b) Photograph of the sample and the distribution of as growth MoO3 under an optical microscope. The enlarged image of A, B, and C are showed in Figure S1 (d)-(f). (c) The X-ray diffraction of flat lying MoO3; (d) HRTEM of MoO3, inset shows the SAED pattern. Figure 3 (a)-(c) AFM images of MoO3 nanosheets with different thickness on SiO2 surface. The insets show the optical images. The line profiles are taken at the black dash lines. (d) The Raman spectra of the MoO3 nanosheets with different thickness. Label “ * ” identify peaks for MoO3 and “ @ ” for SiO2. Figure 4 (a) Typical AFM image of a MoO3 FL device; (b) Dark current of MoO3 FL devices; (c) & (d) photoresponse of FL devices after annealing under illumination of 405 nm and 638 nm respectively. The labels are the laser power with unit of mW. Figure 5 Surface morphology after vacuum annealing: (a) Annealed sample in Figure 4(a). The arrows indicate the line defects; (b) Annealed sample showing three types of defects. (c) Surface potential mapping of the device before and after vacuum annealing and (d) is the corresponding work function distribution on Au electrode and MoO3 calculated from (c). Figure 6 XPS spectroscopy of MoO3 nanoplates before and after annealing (a) Mo 3d5 and 3d3 peaks; (b) valance structure. Figure 7 (a) AFM topography image of a defect along [001] direction; (b) Optical near-field amplitude (O4) image at 633 nm associated with the topography shown in (b). The line profiles correspond to the position of dash line in related images. Figure 8 (a) Surface potential images before and under illumination; (b) The work function distributions of Au electrode and MoO3 in (a); (c) Schematic energy diagram under illumination.

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Figure 1 Schematic structure of α-MoO3: the thickness of one MoO3 layer is 0.7 nm; there are three oxygen sites: Ot, Oa and Os; the breaking of these sites can form three types of line defects 42x12mm (300 x 300 DPI)


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Figure 2 (a) Schematic set-up of the growth process of MoO3 in physical vapor deposition system; (b) Photograph of the sample and the distribution of as growth MoO3 under an optical microscope ; (c) The Xray diffraction of flat lying MoO3; (d) HRTEM of MoO3, inset shows the SAED pattern. 110x122mm (300 x 300 DPI)



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Figure 3 (a)-(c) AFM images of MoO3 nanosheets with different thickness on SiO2 surface. The insets show the optical images. The line profiles are taken at the black dash lines. (d) The Raman spectra of the MoO3 nanosheets with different thickness. Label “ * ” identify peaks for MoO3 and “ @ ” for SiO2. 113x91mm (300 x 300 DPI)


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Figure 4 (a) Typical AFM image of a MoO3 FL device; (b) Dark current of MoO3 FL devices; (c) & (d) photoresponse of FL devices after annealing under illumination of 405 nm and 638 nm respectively. The labels are the laser power with unit of mW. 66x55mm (300 x 300 DPI)



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Figure 5 Surface morphology after vacuum annealing: (a) Annealed sample in Figure 4(a). The arrows indicate the line defects; (b) Annealed sample showing three types of defects. (c) Surface potential mapping of the device before and after vacuum annealing and (d) is the corresponding work function distribution on Au electrode and MoO3 calculated from (c). 78x61mm (300 x 300 DPI)


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Figure 6 XPS spectroscopy of MoO3 nanoplates before and after annealing (a) Mo 3d5 and 3d3 peaks; (b) valance structure. 33x14mm (300 x 300 DPI)



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Figure 7 (a) AFM topography image of a defect along [001] direction; (b) Optical near-field amplitude (O4) image at 633 nm associated with the topography shown in (b). The line profiles correspond to the position of dash line in related images. 55x31mm (300 x 300 DPI)


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Figure 8 (a) Surface potential images before and under illumination; (b) The work function distributions of Au electrode and MoO3 in (a); (c) Schematic energy diagram under illumination. 46x13mm (300 x 300 DPI)



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For Table of Contents Only 35x14mm (600 x 600 DPI)


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Growth of Large-Scale, Large-Size, Few-Layered Î± ... - ACS Publications

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