[Show abstract][Hide abstract] ABSTRACT: Ultrastrong and precisely controllable n-type photoinduced doping at a graphene/TiOx heterostructure as a result of trap-state-mediated charge transfer is demonstrated, which is much higher than any other reported photodoping techniques. Based on the strong light-matter interactions at the graphene/TiOx heterostructure, precisely controlled photoinduced bandgap opening of a bilayer graphene device is demonstrated.
[Show abstract][Hide abstract] ABSTRACT: To replace high-temperature sintered scaffold materials in conventional CH3NH3PbI3-based solar cells, this study demonstrates a new device structure of a bulk intermixing (BI)-type CH3NH3PbI3/TiO2 nanorod (NR) hybrid solar cell, where dispersed TiO2 NRs from chemical synthesis are intermixed with the perovskite absorbing layer to form a BI-type perovskite/TiO2 NR hybrid for device fabrication. Through interface engineering between the TiO2 NR surface and the photoactive perovskite material of CH3NH3PbI3 by ligand exchange treatment, a remarkable power conversion efficiency (PCE) of over 12% was achieved based on the simple BI-type CH3NH3PbI3/TiO2 NR hybrid device structure. The proposed hybrids not only provide great flexibility for deposition on various substrates through spin coating at low temperatures but also enable layer-by-layer deposition for the future development of perovskite-based multi-junction solar cells.
[Show abstract][Hide abstract] ABSTRACT: Interfacial science has received much attention recently based on the development of state-of-the-art analytical tools that can create and manipulate the charge, spin, orbital, and lattice degrees of freedom at interfaces. Motivated by the importance of nanoscale interfacial science that governs device operation, we present a technique to probe the electronic characteristics of heterointerfaces with atomic resolution. In this work, the interfacial characteristics of heteroepitaxial structures are investigated and the fundamental mechanisms that pertain in these systems are elucidated through cross-sectional scanning tunneling microscopy (XSTM). The XSTM technique is employed here to directly observe epitaxial interfacial structures and probe local electronic properties with atomic-level capability. Scanning tunneling microscopy and spectroscopy experiments with atomic precision provide insight into the origin and spatial distribution of electronic properties across heterointerfaces. The first part of this report provides a brief description of the cleavage technique and spectroscopy analysis in XSTM measurements. The second part addresses interfacial electronic structures of several model heterostructures in current condensed matter research using XSTM. Topics to be discussed include high-κ's/III-V's semiconductors, polymer heterojunctions, and complex oxide heterostructures, which are all material systems whose investigation using this technique is expected to benefit the research community. Finally, practical aspects and perspectives of using XSTM in interface science are presented.
[Show abstract][Hide abstract] ABSTRACT: We report semiconducting behavior of monolayer graphene enabled through
plasma activation of substrate surfaces. The graphene devices are fabricated by
mechanical exfoliation onto pre-processed SiO2/Si substrates. Contrary to
pristine graphene, these graphene samples exhibit a transport gap as well as
nonlinear transfer characteristics, a large on/off ratio of 600 at cryogenic
temperatures, and an insulating-like temperature dependence. Raman
spectroscopic characterization shows evidence of sp3 hybridization of C atoms
in the samples of graphene on activated SiO2/Si substrates. We analyze the
hopping transport at low temperatures, and weak localization observed from
magnetotransport measurements, suggesting a correlation between carrier
localization and the sp3-type defects in the functionalized graphene. The
present study demonstrates the functionalization of graphene using a novel
substrate surface-activation method for future graphene-based applications.
[Show abstract][Hide abstract] ABSTRACT: Recent discoveries of the photoresponse of molybdenum disulfide (MoS2) have
shown the considerable potential of these two-dimensional transition metal
dichalcogenides for optoelectronic applications. Among the various types of
photoresponses of MoS2, persistent photoconductivity (PPC) at different levels
has been reported. However, a detailed study of the PPC effect and its
mechanism in MoS2 is still not available, despite the importance of this effect
on the photoresponse of the material. Here, we present a systematic study of
the PPC effect in monolayer MoS2 and conclude that the effect can be attributed
to random localized potential fluctuations in the devices. Notably, the
potential fluctuations originate from extrinsic sources based on the substrate
effect of the PPC. Moreover, we point out a correlation between the PPC effect
in MoS2 and the percolation transport behavior of MoS2. We demonstrate a unique
and efficient means of controlling the PPC effect in monolayer MoS2, which may
offer novel functionalities for MoS2-based optoelectronic applications in the
[Show abstract][Hide abstract] ABSTRACT: Compared to widely-reported graphene-based anode, the task to obtain a stable graphene-based cathode is generally more difficult to achieve because n-type graphene devices have very limited thermal and chemical stabilities, and are usually sensitive to the influence of ambient environment. This work developed a novel “sunlight-activated” graphene-heterostructure transparent electrode in which photogenerated charges from a light-absorbing material are transferred to graphene, resulting in the modulation of electrical properties of the graphene transparent electrode caused by a strong light–matter interaction at graphene-heterostructure interfaces. A photoactive graphene/TiOx-heterostructure transparent cathode was used to fabricate an n-graphene/p-Si Schottky junction solar cell, achieving a record-high power conversion efficiency (>10%). The photoactive graphene-heterostructure transparent electrode, which exhibits excellent tunable electrical properties under sunlight illumination, has great potential for use in the future development of graphene-based photovoltaics and optoelectronics.
[Show abstract][Hide abstract] ABSTRACT: The photoluminescence (PL) quenching of water-soluble graphene oxide (GO) solution was systematically investigated in the presence of transition metal ions. Their PL spectra were analyzed by the Stern-Volmer equation, and the trend of the quenching efficiency was Fe2+ > Co2+ > Ni2+ > Cd2+ > Hg2+. The results of the steady-state and time-resolved PL spectra of the GO solution suggested that the PL quenching was related to the new non-radiative optical transitions from the bridging states due to the hybridization of the sp(3) orbitals of GO and the 3d orbitals of metal ions, proven by density functional theory calculations. The overall results indicated that the bridging states from the hybridization of GO sp(3) and unfilled 3d orbitals (Fe2+) in comparison with filled 3d orbitals (Hg2+) were highly localized, and their energy levels were more suitable for being non-radiative transition states.
[Show abstract][Hide abstract] ABSTRACT: Hydrogen evolution reaction (HER) from water through electrocatalysis using cost-effective materials to replace precious Pt catalysts holds great promise for clean energy technologies. In this work we developed a highly active and stable catalyst containing Co doped earth abundant iron pyrite FeS2 nanosheets hybridized with carbon nanotubes (Fe1-xCoxS2/CNT hybrid catalysts) for HER in acidic solutions. The pyrite phase of Fe1-xCoxS2/CNT was characterized by powder X-ray diffraction and absorption spectroscopy. Electrochemical measurements showed a low overpotential of ~0.12 V at 20 mA/cm2, small Tafel slope of ~ 46 mV/decade and long-term durability over 40 h of HER operation using bulk quantities of Fe0.9Co0.1S2/CNT hybrid catalysts at high loadings (~ 7 mg/cm2). Density functional theory calculation revealed that the origin of high catalytic activity was stemmed from a large reduction of the kinetic energy barrier of H atom adsorption on FeS2 surface upon Co doping in the iron pyrite structure. It is also found that the high HER catalytic activity of Fe0.9Co0.1S2 hinge on the hybridization with CNTs to impart strong heteroatomic interactions between CNT and Fe0.9Co0.1S2. This work produces the most active HER catalyst based on iron pyrite, suggesting a scalable, low cost and highly efficient catalyst for hydrogen generation.
Journal of the American Chemical Society 01/2015; 137(4). DOI:10.1021/ja511572q · 12.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper, we demonstrate an inspection technique-based on only one ellipsometric parameter, Ψ, of spectroscopic ellipsometry (SE) - for the rapid, simultaneous identification of both the structural quality and thicknesses of large-area graphene films. The measured Ψ spectra are strongly affected by changes in the out-of-plane absorption coefficients (αTM); they are also correlated to the ratio of the intensities of the D and G bands in Raman spectra of graphene films. In additional, the electronic transition state of graphene within the UV regime assists the characterization of the structural quality. We also demonstrated that the intensities and shifts of the signals in Ψ spectra allow clear identification of the structural qualities and thicknesses, respectively, of graphene films. Moreover, this Ψ-based method can be further applied to graphene films coated on various substrates. To the best of our knowledge, this paper is the first to describe the direct application of Ψ spectra obtained through conventional SE to determine the structural qualities of graphene films on different substrates (Cu, fused silica, Si). In addition, mapping of the values of Ψ is a very convenient and useful means of rapidly characterizing both the structural quality and thickness of 2D materials at local areas. Therefore, this Ψ-based characterization method has great potential for application in the mass production of devices based on large-area graphene.
[Show abstract][Hide abstract] ABSTRACT: In this study, we systematically investigated the stoichiometric dependence of titanium oxide (TiOx, x=1.56–1.93) as a cathode modifier on the device performance of polymer solar cells. Electronic structures of the synthesized TiOx modifier layers were controlled by tuning the compositions of various O/Ti ratios. The effective cathode work-functions and the corresponding device performances of polymer solar cells are systematically changed as a result of inserting the TiOx modification layers. Interfacial modification of the Al cathode with a low O/Ti ratio of TiOx layer yields the best performing photovoltaic device as a result of a largest built-in potential. The correlation of power conversion efficiencies and carrier dynamics of these devices by inserting various TiOx modification layer is further examined by using the Mott-Schottky analysis and the impedance spectroscopy technique. The consistent result shows an enhanced carrier collection efficiency and a reduced charge recombination rate of the device via adequate band alignment between the photoactive layer and the cathode using the TiOx modification layer with an optimized O/Ti ratio.
Solar Energy Materials and Solar Cells 06/2014; 125:233–238. DOI:10.1016/j.solmat.2014.03.017 · 5.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two-dimensional (2D) atomic crystals and their hybrid structures have recently attracted much attention due to their potential applications. The fabrication of metallic contacts or nanostructures on 2D materials is very common and generally achieved by performing electron-beam (e-beam) lithography. However, e-beam lithography is not applicable in certain situations, e.g., cases in which the e-beam resist does not adhere to the substrates or the intrinsic properties of the 2D materials are greatly altered and degraded. Here, we present a residue-free approach for fabricating high-performance graphene devices by patterning a thin film of e-beam resist as a stencil mask. This technique can be generally applied to substrates with varying surface conditions, while causing negligible residues on graphene. The technique also preserves the design flexibility offered by e-beam lithography and therefore allows us to fabricate multi-probe metallic contacts. The graphene field-effect transistors fabricated by this method exhibit smooth surfaces, high mobility, and distinct magnetotransport properties, confirming the advantages and versatility of the presented residue-free technique for the fabrication of devices composed of 2D materials.
[Show abstract][Hide abstract] ABSTRACT: We report the discovery of superconductivity with an onset temperature of ∼0.6 K in a platinum-silicon interface. The interface was formed by using a unique focused ion beam sputtering micro-deposition method in which the energies of most sputtered Pt atoms are ∼2.5 eV. Structural and elemental analysis by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy reveal a ∼ 7 nm interface layer with abundant Pt, which is the layer likely responsible for the superconducting transport behavior. Similar transport behavior was also observed in a gold-silicon interface prepared by the same technique, indicating the possible generality of this phenomenon.
[Show abstract][Hide abstract] ABSTRACT: This work demonstrated a simple platform for rapid and effective surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) measurements based on the layer structure of reduced graphene oxide (rGO) and gold nanoparticles. A multi-layer thin film was fabricated by alternate layer-by-layer depositions of rGO and gold nanoparticles (LBL rGO/AuNP). The flat and clean two-dimensional film was served as the sample plate and also functioned as the matrix in SALDI-TOF MS. By simply one-step deposition of analytes onto the LBL rGO/AuNP sample plate, the MS measurements of various homogeneous samples were ready to execute. The optimization of MS signal was reached by the variation of the layer numbers of rGO and gold nanoparticles. Also, the small molecules including amino acids, carbohydrates and peptides were successfully analyzed in SALDI-TOF MS using the LBL rGO/AuNP sample plate. The results showed that the signal intensity, SN(-1) ratio and reproducibility of SALDI-TOF spectra have been significantly improved in comparison to the uses of gold nanoparticles or α-cyano-4-hydroxy-cinnamic acid (CHCA) as the assisted matrixes. Taking the advantages of the unique properties of rGO and gold nanoparticles, the ready-to-use MS sample plate, which could absorb and dissipate laser energy to analytes quite efficiently and homogeneously, has shown great commercial potentials for MS applications.
[Show abstract][Hide abstract] ABSTRACT: A unique "clean-lifting transfer" (CLT) technique that applies a controllable electrostatic force to transfer large-area and high-quality CVD-grown graphene onto various rigid or flexible substrates is reported. The CLT technique without using any organic support or adhesives can produce residual-free graphene films with large-area processability, and has great potential for future industrial production of graphene-based electronics or optoelectronics.
[Show abstract][Hide abstract] ABSTRACT: Polymer solar cells have great potential for offering a cost-effective approach for converting solar energy into electricity compared to traditional inorganic counterparts. Besides the most intensively studied materials for polymer solar cells consisting of conducting polymer and fullerene derivative hybrids, polymer–inorganic nanocrystal (NC) hybrid solar cell devices represent promising alternatives by taking advantage of the relatively high electron mobility, good physical and chemical stability and various morphologies of inorganic NCs. This paper presents a review of the current status and development of polymer–inorganic hybrid solar cells based on metal oxide NCs by focusing the discussion on TiO2 and ZnO. These metal oxide NC materials are promising acceptor candidates because they are environmentally friendly and cheap to be synthesized by using wet chemical methods with a wide range of morphologies, enabling full compatibility with the solution-processable fabrication of polymer solar cells. Substantial progress has been achieved recently in the power conversion efficiencies of polymer–metal-oxide hybrid solar cells through the control of nanoscale polymer–inorganic hybrid morphologies and the improved interfaces between polymers and inorganic nanocrystals. We also reviewed the recently developed state-of-the-art analytical techniques introduced to reveal the nanoscale morphological organization of polymers and NCs in polymer–metal-oxide hybrid solar cells, which provides the understanding of the interplay between controlling nanoscale morphologies of polymer–metal-oxide NC hybrids and photocarrier dynamics and the corresponding device performance. Finally, the main challenges in the development of polymer–metal-oxide hybrid solar cells consisting of both bulk heterojunctions (BHJs) and nanostructured hybrid device architectures are identified, and strategies for improving the device performances are also discussed.
[Show abstract][Hide abstract] ABSTRACT: Calligraphic counter electrodes: An important photovoltaic application using FeS2 nanocrystal (NC) pyrite ink to fabricate a counter electrode as an alternative to Pt in dye-sensitized solar cells is demonstrated. FeS2 NC ink exhibits excellent electrochemical catalytic activity and remarkable electrochemical stability. ITO=indium-doped tin oxide.
[Show abstract][Hide abstract] ABSTRACT: We present prominent photoresponse of bio-inspired graphene-based
phototransistors sensitized with chlorophyll molecules. The hybrid
graphene-chlorophyll phototransistors exhibit a high gain of 10^6 electrons per
photon and a high responsivity of 10^6 A/W, which can be attributed to the
integration of high-mobility graphene and the photosensitive chlorophyll
molecules. The charge transfer at interface and the photogating effect in the
chlorophyll layer can account for the observed photoresponse of the hybrid
devices, which is confirmed by the back-gate-tunable photocurrent as well as
the thickness and time dependent studies of the photoresponse. The
demonstration of the graphene-chlorophyll phototransistors with high gain
envisions a viable method to employ biomaterials for graphene-based
[Show abstract][Hide abstract] ABSTRACT: Using cross-sectional scanning tunneling microscope (XSTM) with samples cleaved in situ in an ultrahigh vacuum (UHV) chamber, this study demonstrates the direct visualization of high-resolution interfacial band mapping images across the film thickness in an optimized bulk heterojunction polymer solar cell consisting of nanoscale phase segregated blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We were able to achieve the direct observation of the interfacial band alignments at the donor (P3HT)-acceptor (PCBM) interfaces and at the interfaces between the photoactive P3HT:PCBM blends and the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) anode modification layer with an atomic-scale spatial resolution. The unique advantage of using XSTM to characterize polymer:fullerene bulk heterojunction solar cells allows us to explore simultaneously the quantitative link between the vertical morphologies and their corresponding local electronic properties. This provides an atomic insight of interfacial band alignments between the two opposite electrodes, which will be crucial for improving the efficiencies of the charge generation, transport, and collection and the corresponding device performance of polymer solar cells.