Yung-Huang Chang - Academia.edu (original) (raw)
Papers by Yung-Huang Chang
Advanced Science, Jan 25, 2019
Nature Communications, May 6, 2016
Impressive properties arise from the atomically thin nature of transition metal dichalcogenide tw... more Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe 2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of B20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe 2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters.
International Journal of Hydrogen Energy, 2014
Amorphous molybdenum sulfide (MoS x) materials have been considered as cheap and promising cataly... more Amorphous molybdenum sulfide (MoS x) materials have been considered as cheap and promising catalysts for hydrogen evolution reaction (HER). In this contribution, we report that the amorphous MoS x catalysts prepared by the low temperature thermolysis of the (NH 4) 2 MoS 4 precursors on carbon clothes (catalyst loading: 3.2 mg/cm 2) exhibit a Tefal slope of 50.5 mV/dec and a high exchange current density of 1.5 Â 10 À3 mA/cm 2 in 0.5 M H 2 SO 4 solutions. Spectroscopic studies of the amorphous MoS x catalysts show that the increase of HER efficiency is positively correlated to the concentration of S 2 2À species, providing strong evidence to support the argument that S 2 2À is an active species for electrocatalytic HER. Additionally, the method for preparing catalysts is simple, scalable and applicable for large-scale production.
ACS Applied Materials & Interfaces, 2014
Molybdenum sulfide has recently attracted much attention because of its low cost and excellent ca... more Molybdenum sulfide has recently attracted much attention because of its low cost and excellent catalytical effects in the application of hydrogen evolution reaction (HER). To improve the HER efficiency, many researchers have extensively explored various avenues such as material modification, forming hybrid structures or modifying geometric morphology. In this work, we reported a significant enhancement in the electrocatalytic activity of the MoS x via growing on Tetracyanoquinodimethane (TCNQ) treated carbon cloth, where the MoS x was synthesized by thermolysis from the ammonium tetrathiomolybdate ((NH 4) 2 MoS 4) precursor at 170°C. The pyridinic N-and graphitic N-like species on the surface of carbon cloth arising from the TCNQ treatment facilitate the formation of Mo 5+ and S 2 2− species in the MoS x , especially with S 2 2− serving as an active site for HER. In addition, the smaller particle size of the MoS x grown on TCNQ-treated carbon cloth reveals a high ratio of edge sites relative to basal plane sites, indicating the richer effective reaction sites and superior electrocatalytic characteristics. Hence, we reported a high hydrogen evolution rate for MoS x on TCNQ-treated carbon cloth of 6408 mL g −1 cm −2 h −1 (286 mmol g −1 cm −2 h −1) at an overpotential of V = 0.2 V. This study provides the fundamental concepts useful in the design and preparation of transition metal dichalcogenide catalysts, beneficial in the development in clean energy.
Bulletin of the American Physical Society, 2018
Journal of The Electrochemical Society, 2018
Nanostructures, such as nanodots and nanorods, have been fabricated by the anodization process wi... more Nanostructures, such as nanodots and nanorods, have been fabricated by the anodization process with an aid of anodic aluminum oxide (AAO) for several decades. However, tubular nanostructures have not been reported by using an AAO-assistant anodization technology until now. In this study, we successfully synthesized self-organized Ti x W y O nanotubes with cap structures for the first time by using a two-step anodization process from Al-coated TiW alloy layers. The Ti x W y O nanotubes were observed only in the shaped pores of AAO with large enough pore size, and otherwise the nanostructures would become solid rods due to the aggregation of Ti x W y O. The wall of nanotubes was ∼30% inside the pores and the other ∼70% penetrated into the AAO template. Furthermore, no tubular nanostructures were found unless the percentage of W element in TiW alloys is larger than 50%, attributing to the formation of a void by oxygen bubbles arising from the oxidation reaction of tungsten metal. Besides, the height of the nanotubes is limited by an electric field of 0.82 GV/m due to the dielectric breakdown effect. Finally, a bottom-up growth mechanism at the oxide/metal interface is proposed, based on the observation of the cap structure on the top of the tubular architecture.
Physical Review Letters, 2017
Using wide spectral range in situ spectroscopic ellipsometry with systematic ultra high vacuum an... more Using wide spectral range in situ spectroscopic ellipsometry with systematic ultra high vacuum annealing and in situ exposure to oxygen, we report the complex dielectric function of MoS2 isolating the environmental effects and revealing the crucial role of unpassivated and passivated sulphur vacancies. The spectral weights of the A (1.92 eV) and B (2.02 eV) exciton peaks in the dielectric function reduce significantly upon annealing, accompanied by spectral weight transfer in a broad energy range. Interestingly, the original spectral weights are recovered upon controlled oxygen exposure. This tunability of the excitonic effects is likely due to passivation and reemergence of the gap states in the bandstructure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. This work unravels and emphasizes the important role of adsorbed oxygen in the optical spectra and many-body interactions of MoS2.
Advanced Functional Materials, 2016
require type II HJs for light harvesting, [4-6] and light-emitting diodes benefit from multiple q... more require type II HJs for light harvesting, [4-6] and light-emitting diodes benefit from multiple quantum wells with the type I band alignment for high emission efficiency. [7-9] The vertical tunneling field effect transistor for next-generation electronics counts on nearly broken-gap band alignment for boosting up its performance. [10-12] However, the HJ devices based on conventional bulk materials are severely dominated by several factors which are hard to control such as lattice mismatch and interfacial defects. [13,14] This interfacial defects may impose more significant impacts on the HJs based on an ultrathin channel. [15] Atomically thin 2D transition metal dichalcogenide (TMD) materials have brought revolutionized concepts in constructing devices with van der Waals interaction between monolayers. [16,17] These monolayers can be stacked with arbitrary order and orientation providing various novel HJs. [18] However, to date, the band-alignment for the vertical HJs constructed by two arbitrary 2D large-scale chemical vapor deposition (CVD) films still remains unclear. To quantify band offset on specific two or three layered HJs, computational method [19-24] or the experimental measurement as we reported earlier [25] is time consuming, costly, or relies on critical sample preparation procedures. Therefore, a workable model needs to be urgently identified for this fast growing research field. Anderson proposed that the band alignment in ideal bulk material HJ systems should be predicted by Anderson's model, or named electron affinity rule (EAR). [26] Base on the principle that the vacuum level lines up for the two materials after contact, electron affinity difference for the two materials thus gives a conduction band offset (CBO) and then the valence band offset (VBO) can be deduced by the bandgap difference. Yet, this model fails to predict the band alignment accurately in most cases because of the Fermi level pinning effect induced by charge transfer at the interfacial bonding. [27-30] Considering the unique nature of 2D materials where the weak van der Waals interface is atomically abrupt with no dangling bond, [31] is Anderson's model valid for 2D vertical HJs? In this work, we exam the feasibility of Anderson's model on 2D vertical HJs. We start with the measurements of the valence band maximum (VBM) with respect to vacuum level for each individual TMD films (MoS 2 , WSe 2 , and WS 2) as we reported earlier, [25] followed by constructing the band alignment based on Anderson's model. The predicted band alignments
Nano letters, Jun 9, 2016
Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties ... more Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6°. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport...
Nature communications, May 6, 2016
Impressive properties arise from the atomically thin nature of transition metal dichalcogenide tw... more Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of ∼20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously des...
ACS nano, Jan 20, 2016
The nature and extent of electronic screening at heterointerfaces and their consequences on energ... more The nature and extent of electronic screening at heterointerfaces and their consequences on energy level alignment are of profound importance in numerous applications, such as solar cells, electronics etc. The increasing availability of two-dimensional (2D) transition metal dichalcogenides (TMDs) brings additional opportunities for them to be used as interlayers in "van der Waals (vdW) heterostructures" and organic/inorganic flexible devices. These innovations raise the question of the extent to which the 2D TMDs participate actively in dielectric screening at the interface. Here we study perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) monolayers adsorbed on single-layer tungsten diselenide (WSe2), bare graphite and Au(111) surfaces, revealing a strong dependence of the PTCDA HOMO-LUMO gap on the electronic screening effects from the substrate. The monolayer WSe2 interlayer provides substantial - but not complete - screening at the organic/inorganic interface. Our re...
Advanced Energy Materials, 2014
Scientific reports, Jan 23, 2014
Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic... more Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 10(8). Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, ...
ACS nano, Jan 26, 2014
Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics fo... more Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful ...
Nanotechnology, 2010
This study investigated Schottky-and ohmic-contact effects upon the photoresponses of ITO/TiO 2 /... more This study investigated Schottky-and ohmic-contact effects upon the photoresponses of ITO/TiO 2 /Si and Ti/TiO 2 /Si nanotube-based photodiodes. The TiO 2 tube arrays were fabricated by atomic layer deposition (ALD) and shaped by an anodic aluminum oxide (AAO) template on a p-type Si substrate. The contact area between the electrode (Ti or ITO) and the TiO 2 's tip was varied by tuning the tube's inner wall thickness with ALD, providing a direct and systematic probe of the heterojunction effects upon the photodiodes' responses. Results show that the Ti/TiO 2 /Si diode exhibits a highly thickness-dependent photoresponse. This is because the photocurrent is driven by the p-n junction at TiO 2 /Si alone and it faces no retarding at the ohmic contact of Ti/TiO 2. For the ITO/TiO 2 /Si diode, the Schottky contact at ITO/TiO 2 regulates photocurrent overriding TiO 2 /Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with the Ti/TiO 2 /Si diode. Respective energy band diagrams are provided to support the statements above, and a consistent picture is obtained for both time response and quantum efficiency measurements.
Nanoscale Research Letters, 2012
The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated... more The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated. For ITO/TiO2/Si diodes, the photocurrent is controlled either by the TiO2/Si heterojunction (p-n junction) or the ITO-TiO2 heterojunction (Schottky contact). In the short circuit (approximately 0 V) condition, the TiO2-Si heterojunction dominates the photocarrier transportation direction due to its larger space-charge region and potential gradient. The detailed transition process of the photocarrier direction was investigated with a time-dependent photoresponse study. The results showed that the diode transitioned from TiO2-Si heterojunction-controlled to ITO-TiO2 heterojunction-controlled as we applied biases from approximately 0 to -1 V on the ITO electrode.
Advanced Science, Jan 25, 2019
Nature Communications, May 6, 2016
Impressive properties arise from the atomically thin nature of transition metal dichalcogenide tw... more Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe 2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of B20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe 2 with judiciously designed plasmonic nanostructures and paves a way towards the implementation of plasmon-enhanced transition metal dichalcogenide photodetectors, sensors and emitters.
International Journal of Hydrogen Energy, 2014
Amorphous molybdenum sulfide (MoS x) materials have been considered as cheap and promising cataly... more Amorphous molybdenum sulfide (MoS x) materials have been considered as cheap and promising catalysts for hydrogen evolution reaction (HER). In this contribution, we report that the amorphous MoS x catalysts prepared by the low temperature thermolysis of the (NH 4) 2 MoS 4 precursors on carbon clothes (catalyst loading: 3.2 mg/cm 2) exhibit a Tefal slope of 50.5 mV/dec and a high exchange current density of 1.5 Â 10 À3 mA/cm 2 in 0.5 M H 2 SO 4 solutions. Spectroscopic studies of the amorphous MoS x catalysts show that the increase of HER efficiency is positively correlated to the concentration of S 2 2À species, providing strong evidence to support the argument that S 2 2À is an active species for electrocatalytic HER. Additionally, the method for preparing catalysts is simple, scalable and applicable for large-scale production.
ACS Applied Materials & Interfaces, 2014
Molybdenum sulfide has recently attracted much attention because of its low cost and excellent ca... more Molybdenum sulfide has recently attracted much attention because of its low cost and excellent catalytical effects in the application of hydrogen evolution reaction (HER). To improve the HER efficiency, many researchers have extensively explored various avenues such as material modification, forming hybrid structures or modifying geometric morphology. In this work, we reported a significant enhancement in the electrocatalytic activity of the MoS x via growing on Tetracyanoquinodimethane (TCNQ) treated carbon cloth, where the MoS x was synthesized by thermolysis from the ammonium tetrathiomolybdate ((NH 4) 2 MoS 4) precursor at 170°C. The pyridinic N-and graphitic N-like species on the surface of carbon cloth arising from the TCNQ treatment facilitate the formation of Mo 5+ and S 2 2− species in the MoS x , especially with S 2 2− serving as an active site for HER. In addition, the smaller particle size of the MoS x grown on TCNQ-treated carbon cloth reveals a high ratio of edge sites relative to basal plane sites, indicating the richer effective reaction sites and superior electrocatalytic characteristics. Hence, we reported a high hydrogen evolution rate for MoS x on TCNQ-treated carbon cloth of 6408 mL g −1 cm −2 h −1 (286 mmol g −1 cm −2 h −1) at an overpotential of V = 0.2 V. This study provides the fundamental concepts useful in the design and preparation of transition metal dichalcogenide catalysts, beneficial in the development in clean energy.
Bulletin of the American Physical Society, 2018
Journal of The Electrochemical Society, 2018
Nanostructures, such as nanodots and nanorods, have been fabricated by the anodization process wi... more Nanostructures, such as nanodots and nanorods, have been fabricated by the anodization process with an aid of anodic aluminum oxide (AAO) for several decades. However, tubular nanostructures have not been reported by using an AAO-assistant anodization technology until now. In this study, we successfully synthesized self-organized Ti x W y O nanotubes with cap structures for the first time by using a two-step anodization process from Al-coated TiW alloy layers. The Ti x W y O nanotubes were observed only in the shaped pores of AAO with large enough pore size, and otherwise the nanostructures would become solid rods due to the aggregation of Ti x W y O. The wall of nanotubes was ∼30% inside the pores and the other ∼70% penetrated into the AAO template. Furthermore, no tubular nanostructures were found unless the percentage of W element in TiW alloys is larger than 50%, attributing to the formation of a void by oxygen bubbles arising from the oxidation reaction of tungsten metal. Besides, the height of the nanotubes is limited by an electric field of 0.82 GV/m due to the dielectric breakdown effect. Finally, a bottom-up growth mechanism at the oxide/metal interface is proposed, based on the observation of the cap structure on the top of the tubular architecture.
Physical Review Letters, 2017
Using wide spectral range in situ spectroscopic ellipsometry with systematic ultra high vacuum an... more Using wide spectral range in situ spectroscopic ellipsometry with systematic ultra high vacuum annealing and in situ exposure to oxygen, we report the complex dielectric function of MoS2 isolating the environmental effects and revealing the crucial role of unpassivated and passivated sulphur vacancies. The spectral weights of the A (1.92 eV) and B (2.02 eV) exciton peaks in the dielectric function reduce significantly upon annealing, accompanied by spectral weight transfer in a broad energy range. Interestingly, the original spectral weights are recovered upon controlled oxygen exposure. This tunability of the excitonic effects is likely due to passivation and reemergence of the gap states in the bandstructure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. This work unravels and emphasizes the important role of adsorbed oxygen in the optical spectra and many-body interactions of MoS2.
Advanced Functional Materials, 2016
require type II HJs for light harvesting, [4-6] and light-emitting diodes benefit from multiple q... more require type II HJs for light harvesting, [4-6] and light-emitting diodes benefit from multiple quantum wells with the type I band alignment for high emission efficiency. [7-9] The vertical tunneling field effect transistor for next-generation electronics counts on nearly broken-gap band alignment for boosting up its performance. [10-12] However, the HJ devices based on conventional bulk materials are severely dominated by several factors which are hard to control such as lattice mismatch and interfacial defects. [13,14] This interfacial defects may impose more significant impacts on the HJs based on an ultrathin channel. [15] Atomically thin 2D transition metal dichalcogenide (TMD) materials have brought revolutionized concepts in constructing devices with van der Waals interaction between monolayers. [16,17] These monolayers can be stacked with arbitrary order and orientation providing various novel HJs. [18] However, to date, the band-alignment for the vertical HJs constructed by two arbitrary 2D large-scale chemical vapor deposition (CVD) films still remains unclear. To quantify band offset on specific two or three layered HJs, computational method [19-24] or the experimental measurement as we reported earlier [25] is time consuming, costly, or relies on critical sample preparation procedures. Therefore, a workable model needs to be urgently identified for this fast growing research field. Anderson proposed that the band alignment in ideal bulk material HJ systems should be predicted by Anderson's model, or named electron affinity rule (EAR). [26] Base on the principle that the vacuum level lines up for the two materials after contact, electron affinity difference for the two materials thus gives a conduction band offset (CBO) and then the valence band offset (VBO) can be deduced by the bandgap difference. Yet, this model fails to predict the band alignment accurately in most cases because of the Fermi level pinning effect induced by charge transfer at the interfacial bonding. [27-30] Considering the unique nature of 2D materials where the weak van der Waals interface is atomically abrupt with no dangling bond, [31] is Anderson's model valid for 2D vertical HJs? In this work, we exam the feasibility of Anderson's model on 2D vertical HJs. We start with the measurements of the valence band maximum (VBM) with respect to vacuum level for each individual TMD films (MoS 2 , WSe 2 , and WS 2) as we reported earlier, [25] followed by constructing the band alignment based on Anderson's model. The predicted band alignments
Nano letters, Jun 9, 2016
Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties ... more Two-dimensional (2D) transition metal dichalcogenides (TMDs) have revealed many novel properties of interest to future device applications. In particular, the presence of grain boundaries (GBs) can significantly influence the material properties of 2D TMDs. However, direct characterization of the electronic properties of the GB defects at the atomic scale remains extremely challenging. In this study, we employ scanning tunneling microscopy and spectroscopy to investigate the atomic and electronic structure of low-angle GBs of monolayer tungsten diselenide (WSe2) with misorientation angles of 3-6°. Butterfly features are observed along the GBs, with the periodicity depending on the misorientation angle. Density functional theory calculations show that these butterfly features correspond to gap states that arise in tetragonal dislocation cores and extend to distorted six-membered rings around the dislocation core. Understanding the nature of GB defects and their influence on transport...
Nature communications, May 6, 2016
Impressive properties arise from the atomically thin nature of transition metal dichalcogenide tw... more Impressive properties arise from the atomically thin nature of transition metal dichalcogenide two-dimensional materials. However, being atomically thin limits their optical absorption or emission. Hence, enhancing their photoluminescence by plasmonic nanostructures is critical for integrating these materials in optoelectronic and photonic devices. Typical photoluminescence enhancement from transition metal dichalcogenides is 100-fold, with recent enhancement of 1,000-fold achieved by simultaneously enhancing absorption, emission and directionality of the system. By suspending WSe2 flakes onto sub-20-nm-wide trenches in gold substrate, we report a giant photoluminescence enhancement of ∼20,000-fold. It is attributed to an enhanced absorption of the pump laser due to the lateral gap plasmons confined in the trenches and the enhanced Purcell factor by the plasmonic nanostructure. This work demonstrates the feasibility of giant photoluminescence enhancement in WSe2 with judiciously des...
ACS nano, Jan 20, 2016
The nature and extent of electronic screening at heterointerfaces and their consequences on energ... more The nature and extent of electronic screening at heterointerfaces and their consequences on energy level alignment are of profound importance in numerous applications, such as solar cells, electronics etc. The increasing availability of two-dimensional (2D) transition metal dichalcogenides (TMDs) brings additional opportunities for them to be used as interlayers in "van der Waals (vdW) heterostructures" and organic/inorganic flexible devices. These innovations raise the question of the extent to which the 2D TMDs participate actively in dielectric screening at the interface. Here we study perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) monolayers adsorbed on single-layer tungsten diselenide (WSe2), bare graphite and Au(111) surfaces, revealing a strong dependence of the PTCDA HOMO-LUMO gap on the electronic screening effects from the substrate. The monolayer WSe2 interlayer provides substantial - but not complete - screening at the organic/inorganic interface. Our re...
Advanced Energy Materials, 2014
Scientific reports, Jan 23, 2014
Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic... more Due to its high carrier mobility, broadband absorption, and fast response time, the semi-metallic graphene is attractive for optoelectronics. Another two-dimensional semiconducting material molybdenum disulfide (MoS2) is also known as light- sensitive. Here we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method and graphene is transferable onto MoS2. We demonstrate that a photodetector based on the graphene/MoS2 heterostructure is able to provide a high photogain greater than 10(8). Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the presence of a perpendicular effective electric field caused by the combination of the built-in electric field, ...
ACS nano, Jan 26, 2014
Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics fo... more Monolayer molybdenum disulfide (MoS2) has become a promising building block in optoelectronics for its high photosensitivity. However, sulfur vacancies and other defects significantly affect the electrical and optoelectronic properties of monolayer MoS2 devices. Here, highly crystalline molybdenum diselenide (MoSe2) monolayers have been successfully synthesized by the chemical vapor deposition (CVD) method. Low-temperature photoluminescence comparison for MoS2 and MoSe2 monolayers reveals that the MoSe2 monolayer shows a much weaker bound exciton peak; hence, the phototransistor based on MoSe2 presents a much faster response time (<25 ms) than the corresponding 30 s for the CVD MoS2 monolayer at room temperature in ambient conditions. The images obtained from transmission electron microscopy indicate that the MoSe exhibits fewer defects than MoS2. This work provides the fundamental understanding for the differences in optoelectronic behaviors between MoSe2 and MoS2 and is useful ...
Nanotechnology, 2010
This study investigated Schottky-and ohmic-contact effects upon the photoresponses of ITO/TiO 2 /... more This study investigated Schottky-and ohmic-contact effects upon the photoresponses of ITO/TiO 2 /Si and Ti/TiO 2 /Si nanotube-based photodiodes. The TiO 2 tube arrays were fabricated by atomic layer deposition (ALD) and shaped by an anodic aluminum oxide (AAO) template on a p-type Si substrate. The contact area between the electrode (Ti or ITO) and the TiO 2 's tip was varied by tuning the tube's inner wall thickness with ALD, providing a direct and systematic probe of the heterojunction effects upon the photodiodes' responses. Results show that the Ti/TiO 2 /Si diode exhibits a highly thickness-dependent photoresponse. This is because the photocurrent is driven by the p-n junction at TiO 2 /Si alone and it faces no retarding at the ohmic contact of Ti/TiO 2. For the ITO/TiO 2 /Si diode, the Schottky contact at ITO/TiO 2 regulates photocurrent overriding TiO 2 /Si as a result of higher efficiency in photogeneration, leading to the opposite response compared with the Ti/TiO 2 /Si diode. Respective energy band diagrams are provided to support the statements above, and a consistent picture is obtained for both time response and quantum efficiency measurements.
Nanoscale Research Letters, 2012
The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated... more The heterojunction effects of TiO2 nanotubes on photoconductive characteristics were investigated. For ITO/TiO2/Si diodes, the photocurrent is controlled either by the TiO2/Si heterojunction (p-n junction) or the ITO-TiO2 heterojunction (Schottky contact). In the short circuit (approximately 0 V) condition, the TiO2-Si heterojunction dominates the photocarrier transportation direction due to its larger space-charge region and potential gradient. The detailed transition process of the photocarrier direction was investigated with a time-dependent photoresponse study. The results showed that the diode transitioned from TiO2-Si heterojunction-controlled to ITO-TiO2 heterojunction-controlled as we applied biases from approximately 0 to -1 V on the ITO electrode.