Thickness-dependence of optical constants for Ta2O5 ultrathin films

An effective method for determining the optical constants of Ta₂O₅ thin films deposited on crystal silicon (c-Si) using spectroscopic ellipsometry (SE) measurement with a two-film model (ambient–oxide–interlayer–substrate) was presented. Ta₂O₅ thin films with thickness range of 1–400 nm have been prepared by the electron beam evaporation (EBE) method. We find that the refractive indices of Ta₂O₅ ultrathin films less than 40 nm drop with the decreasing thickness, while the other ones are close to those of bulk Ta₂O₅. This phenomenon was due to the existence of an interfacial oxide region and the surface roughness of the film, which was confirmed by the measurement of atomic force microscopy (AFM). Optical properties of ultrathin film varying with the thickness are useful for the design and manufacture of nano-scaled thin-film devices.     

Investigation of the electronic properties of Au nanoclusters in SiO<Subscript>2</Subscript> by combined scanning tunneling/atomic force microscopy

The tunneling of electrons through Au nanoc lusters formed by pulsed laser deposition in a SiO₂ thin film on a Si substrate has been investigated by combined scanning/atomic force microscopy (STM/AFM). Conducting Pt-coated Si cantilevers were used. The feedback was maintained via the AFM channel, and the current-voltage (I-V) characteristics of the tunnel contact between the AFM probe and the n ⁺-Si substrate through a =4-nm-thick SiO₂ film with Au nanoclusters =2 nm in diameter were measured simultaneously. The current image of the structure contained areas of increased current (tunnel-current channels) 2–15 nm in size, related to tunneling of electrons through Au nanoclusters in SiO₂. The I-V characteristics recorded in the tunnel-current channels exhibit specific features related to the Coulomb blockade of electron tunneling through Au nanoclusters.     

Surface electromigration of In on vicinal Si(0 0 1)

Surface mass transport of In film on vicinal Si(0 0 1) has been systematically investigated by a scanning Auger electron microscopy (SAM), low energy electron diffraction (LEED) and atomic force microscopy (AFM). It was observed that the temperature dependence of the mass transport shows the critical phenomenon. Above a critical temperature T_c, surface electromigration of the In film toward the cathode side dominated the surface mass transport on the vicinal Si(0 0 1) surface. The LEED and AFM observations revealed that the In film surface on the vicinal Si(0 0 1) consists of 3×4 terraces and (3 1 0) facets. The area ratio of the facet to the terrace exhibited abrupt an increase at T_c. It is believed that the change of the mass transport is related to the abrupt change of the area ratio of the facet to the terrace. Both the critical temperature T_c and the spread due to the surface electromigration of the In film depended on the configuration of the DC current direction and the step edge.     

Locally enhanced light–matter interaction of MoS2 monolayers at density-controllable nanogrooves of template-stripped Ag films

Transition metal dichalcogenide (TMD) monolayers, such as MoS_2, possess a direct optical bandgap are useful for emerging ultrathin optoelectronics in the visible light range, whereas their thin thickness limits light absorption and emission properties. To address this drawback, one promising approach is to hybridize plasmonic nanostructures with monolayer TMDs to utilize local field enhancement effects owing to localized surface plasmon resonance (LSPR). Herein, we propose a strong enhancement of the local light–matter interaction in MoS₂ monolayers on naturally generated nanoscale grooves. The nanogrooves are formed at grain boundaries (GBs) of template-stripped metal film substrates that are fabricated by mechanically stripping Ag films deposited on an ultra-flat Si substrate, wherein the nanogroove densities are systematically modulated by the Ag film thickness. We observe an effective photoluminescence enhancement factor of 758 and a Raman spectroscopy intensity enhancement of approximately 5 times in MoS₂ on the subwavelength-scale nanogrooves, compared with that on grain planes, which is attributed to a strong local field enhancement of the LSPR effect. Moreover, this plasmonic enhancement effect is elucidated by dark-field scattering spectroscopy and optical simulations. Our results can facilitate the utilization of density-controllable plasmonic nanogrooves synthesized without nanopatterning techniques for plasmonic hybrids on 2D semiconductors.     

Photodeposition of ultrathin MoS<Subscript>2</Subscript> nanosheets onto cubic CdS for efficient photocatalytic H<Subscript>2</Subscript> evolution

In this work, the photocatalyst composed of ultrathin MoS₂ nanosheets onto the surface of cubic CdS nanoparticles with an average diameter of 7~10 nm has been successfully fabricated through a facile and mild photodeposition route. The ultrathin MoS₂ nanosheets as a cocatalyst were demonstrated to greatly boost photocatalytic H₂ evolution over cubic CdS upon visible light irradiation. It was clearly revealed that both the cubic CdS substrate and structure of ultrathin MoS₂ nanosheets play critical roles in the observed efficient H₂ evolution. The cubic CdS offers a strong adherence for ultrathin MoS₂ nanosheets to form a well contact interface, across which the photogenerated charge transfer and charge separation are achieved. The ultrathin MoS₂ nanosheets introduce a high density of unsaturated active S atoms for H₂ evolution.     

Surface morphology characterization of pentacene thin film and its substrate with under-layers by power spectral density using fast Fourier transform algorithms

Surface morphology of pentacene thin films and their substrates with under-layers is characterized by using atomic force microscopy (AFM). The power values of power spectral density (PSD) for the AFM digital data were determined by the fast Fourier transform (FFT) algorithms instead of the root-mean-square (rms) and peak-to-valley value. The PSD plots of pentacene films on glass substrate are successfully approximated by the k-correlation model. The pentacene film growth is interpreted the intermediation of the bulk and surface diffusion by parameter C of k-correlation model. The PSD plots of pentacene film on Au under-layer is approximated by using the linear continuum model (LCM) instead of the combination model of the k-correlation model and Gaussian function. The PSD plots of SiO₂ layer on Au under-layer as a gate insulator on a gate electrode of organic thin film transistors (OTFTs) have three power values of PSD. It is interpreted that the specific three PSD power values are caused by the planarization of the smooth SiO₂ layer to rough Au under-layer.     

Effect of Intercalated LiF on Interface Contact Characteristics of MoS2 Field Effect Transistor: A First-Principles Study

Based on the first-principles calculation, the effect of intercalated LiF on the contact characteristics of the interface between Au electrode and MoS₂ layer is studied. It is found that adding LiF film can change the contact type between metal electrode Au and MoS₂ layer from Schottky contact to ohmic contact, which is accompanied by interfacial charge transfer from LiF layer to MoS₂ layer and the downward movement of d (d_xy and d_z2) orbital of Mo atom and p (p_x and p_y) orbital of S atom to Fermi level. And the interlayer spacing between LiF layer and Au electrode has a great impact on the interface contact characteristics. The electric field effect and stress effect of interface contact of Au, LiF and MoS₂ (Au/LiF/MoS₂)is more obvious than that of interface contact of Au and MoS₂ (Au/MoS₂). Au/LiF/MoS₂ shows ohmic contact with the interlayer spacing between Au layer and LiF layer less than 3.05 Å and with the electric field less than 0.15 VÅ⁻¹, respectively, while Au/MoS₂ still shows N-type Schottky contact. These findings are helpful to control the contact resistance and have guiding significance for high performance MoS₂ field effect transistor and other electronic components.     

Emission properties of sequentially deposited ultrathin CH3NH3PbI3/MoS2 heterostructures

Hybrid organic-inorganic perovskite materials have obtained considerable attention due to their exotic optoelectronic properties and extraordinarily high performance in photovoltaic devices. Herein, we successively converted the ultrathin PbI₂/MoS₂ into the CH₃NH₃PbI₃/MoS₂ heterostructures via CH₃NH₃I vapor processing. Atomic force microscopy (AFM)、Scanning electron microscopy (SEM) and X-ray photoemission spectroscopy (XPS) measurements prove the high-quality of the converted CH₃NH₃PbI₃/MoS₂. Both MoS₂ and CH₃NH₃PbI₃ related photoluminescence (PL) intensity quenching in CH₃NH₃PbI₃/MoS₂ implies a Type-II energy level alignment at the interface. Temperature-dependent PL measurements show that the emission peak position shifting trend of CH₃NH₃PbI₃ is opposite to that of MoS₂ (traditional semiconductors) due to the thermal expansion and electron-phonon coupling effects. The CH₃NH₃PbI₃/TMDC heterostructures are useful in fabricating innovative devices for wider optoelectronic applications.     

Photovoltaic effect of TiO2 thick films with an ultrathin BiFeO3 as buffer layer

The photovoltaic (PV) effect of a bilayer anatase TiO₂/BiFeO₃ (BFO) film has been studied. The 20-nm ultrathin BFO layers were deposited on the fluorine-doped tin oxide (FTO) glass substrates by the chemical solution deposition method. An anatase TiO₂ layer is deposited subsequently on the BFO surface via a screen-printing technique. It is found that the FTO/TiO₂/Au cell exhibits negligible PV effect under solar exposure, while the one after introducing an ultrathin BFO film between TiO₂ and FTO leads to a considerable PV effect with an open-circuit voltage of ?0.58 V and a photocurrent density of 18.27 µA/cm². The FTO/BiVO₄ (BVO)/TiO₂/Au cell was constructed to investigate the underlying mechanism for the observed effect. A negligible PV effect of the FTO/BVO/TiO₂/Au cell indicates that the PV effect of the FTO/BFO/TiO₂/Au cell arises mainly from a built-in electric field in the BFO film induced by the self-polarization. Our work opens up a new path to utilize TiO₂ and may influence the future design of solar cells.     

Characterization of K2CO3/Co–MoS2 catalyst by XRD,XPS, SEM,and EDS

MoS₂, Co–MoS₂ and K₂CO₃/Co–MoS₂ catalysts have been characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD analysis indicates that Co–MoS₂ is a primary phase in K₂CO₃/Co–MoS₂ catalyst and the diffraction lines of Co–MoS₂ are not changed by the addition of K₂CO₃. Co₉S₈ phase is not present at Co/Mo mole ratio of 0.5 using a co-precipitation method for preparation of cobalt–molybdenum catalyst. The binding energies (BEs) of chemical species present on the surface of the catalysts are compared through the course of catalyst preparation. K₂CO₃/Co–MoS₂ catalyst has been investigated as a function of dispersion of K on the surface and exposure to a mixture of carbon monoxide and hydrogen (syngas) by scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The distribution of potassium on the surface of the K-promoted catalyst is not uniform.     

Molecular beam epitaxy of GaN on a substrate of MoS2 layered compound

The feasibility of MoS₂ layered compound as a substrate for GaN growth was investigated. GaN films were successfully grown on MoS₂ by plasma-enhanced molecular beam epitaxy and the crystal quality of GaN on MoS₂ was compared with that on Al₂O₃. For GaN grown on MoS₂ substrate, it was found that the surface flatness observed by atomic force microscopy, stress in the film measured by Raman spectroscopy, optical properties measured by photoluminescence spectroscopy, and threading dislocation density observed by transmission electron microscopy show superior properties compared with that grown on Al₂O₃. These results suggest the layered compound such as MoS₂, which has no dangling bonds on the surface and has lattice mismatching of 0.9% to GaN, has high potential for a substrate of GaN growth. Received: 1 March 1999 / Accepted: 8 March 1999 / Published online: 26 May 1999     

Enhanced photoluminescence of MoS2–Au nanostructures: Nanotriangle and nanohole arrays

We investigated the photoluminescence (PL) characteristics of MoS₂–Au hybrid nanostructures, fabricated by nanosphere lithography and wet-transfer techniques. Two kinds of Au nanostructures - such as nanotriangles (NTs) and nanoholes (NHs) - were fabricated for comparison. MoS₂ monolayers on both NT and NH arrays exhibited enhanced PL intensity, compared with those on SiO₂/Si substrates and flat Au thin films. Numerical simulations revealed clear distinction in the electric field intensity distributions in the NT and NH arrays at the PL excitation wavelength. Such difference could be attributed to the excitation of localized and propagating surface plasmon in the NT and NH arrays. This work helps us to understand how the plasmonic NT and NH arrays affect the physical properties of the MoS₂ monolayers on them.     

Characteristics of a type-II n-MoS2/p-Ge van der Waals heterojunction

A few-atomic-layer molybdenum disulfide (MoS₂) film on Si/SiO₂ substrates grown by metal-organic chemical vapor deposition was investigated. The few-atomic-layer MoS₂ film was subsequently transferred onto a (100) p-Ge substrate to build a van der Waals n-p heterojunction. The as-grown few-atomic-layer MoS₂ film and the MoS₂/Ge heterostructure were characterized atomic force microscopy, spectroscopic ellipsometry, high-resolution scanning transmission electron microscopy, Raman spectroscopy analyses, photoluminescence (PL) measurements at room temperature (RT, 300 K), and type-II band alignment of the heterostructure determined by ultraviolet photoelectron spectroscopy. The RT-PL measurements showed dominant peaks at 1.96 and 1.8 eV for the as-grown MoS₂ and red-shifted PL peaks for that transferred onto Ge. We examined the electrical characteristics of the few-atomic-layer MoS₂ by forming a type-II band alignment van der Waals heterojunction with a highly doped p-Ge. The heterojunction solar cell exhibited an open-circuit voltage of 0.15 V and a short-circuit current density of 45.26 μA/cm². The external quantum efficiency measurements showed a spectral response up to approximately 500 nm owing to the absorption by the few-atomic-layer MoS₂ film.     

Study of resonant tunneling in Au nanoclusters on the surface of SiO2/Si thin films using the combined scanning tunneling microscopy and atomic-force microscopy technique

The morphology and electronic properties of Au nanoclusters on the surface of SiO₂ thin films on n ⁺-Si substrates are studied using the combined scanning tunneling microscopy (STM) and atomic-force microscopy (AFM) technique. The peaks associated with the resonant tunneling of electrons from the states of the valence band of the probe material to the states of the conduction band of the substrate material through Au nanoclusters are observed on the current-voltage characteristics for the contact of a p ⁺-Si AFM probe with Au nanoclusters. Experimental results are interpreted by calculating the tunnel transparency of the SiO₂/Au/SiO₂ double barrier structure in a strong electric field.     

Deposition of Au/TiO2 film by pulsed laser

Au nanoparticles, which were photoreduced by a Nd:YAG laser in HAuCl₄ solution containing TiO₂ colloid and accompanied by the TiO₂ particles, were deposited on the substrate surface. The film consisting of Au/TiO₂ particles was characterized by the absorption spectra, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The adhesion between the film and substrate was evaluated by using adhesive tape test. It was found that the presence of TiO₂ dramatically enhanced the adhesion strength between the film and the substrate, as well as the deposition rate of film. The mechanism for the deposition of Au/TiO₂ film was also discussed.     

Formation of gold nanoparticles in heat-treated reactive co-sputtered Au-SiO2 thin films

In this work, formation of gold nanoparticles in radio frequency (RF) reactive magnetron co-sputtered Au-SiO₂ thin films post annealed at different temperatures in Ar + H₂ atmosphere has been investigated. Optical, surface topography, chemical state and crystalline properties of the prepared films were analyzed by using UV-visible spectrophotometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD) techniques, respectively. Optical absorption spectrum of the Au-SiO₂ thin films annealed at 800 °C showed one surface plasmon resonance (SPR) absorption peak located at 520 nm relating to gold nanoparticles. According to XPS analysis, it was found that the gold nanoparticles had a tendency to accumulate on surface of the heat-treated films in the metallic state. AFM images showed that the nanoparticles were uniformly distributed on the film surface with grain size of about 30 nm. Using XRD analysis average crystalline size of the Au particles was estimated to about 20 nm.     

Structural and compositional mapping of a phase-separated Langmuir–Blodgett monolayer by X-ray photoelectron emission microscopy

The structure and composition of phase-separated Langmuir–Blodgett monolayer films comprised of mixtures of arachidic acid (C₁₉H₃₉COOH) and perfluorotetradecanoic acid (C₁₃F₂₇COOH) were characterized using a combination of X-ray photoelectron emission microscopy (X-PEEM), secondary electron emission microscopy (SEEM) and atomic force microscopy (AFM). X-PEEM provides high lateral spatial resolution and is directly sensitive to the elemental and chemical (functional group) composition of these ultrathin films through the chemical sensitivity of NEXAFS spectroscopy; AFM provides high-resolution imaging, both in terms of lateral and vertical (height) film topography. SEEM provides additional structural and electronic information through work function and electron scattering effects. The combination is used for chemical mapping of the phase-separated domains in the monolayer film. Our results directly confirm previous AFM measurements that suggested that the discontinuous domains are enriched in arachidic acid, whereas the surrounding continuous domain is a mixture of both arachidic acid and perfluorotetradecanoic acid.     

Electron-assisted chemical etching of oxidized chromium

Electron-assisted chemical etching of oxidized chromium, CrO_x, has been studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM). Two model substrates were used—10 nm CrO_x deposited on Si(1 0 0) that was covered with either native oxide or a 20 nm Au/Pd alloy film. Using chlorine and/or oxygen as etching gases, the experiments were conducted in a customized high vacuum system, equipped with a high density electron source and a low pressure reaction cell. On both substrates, electron-assisted chemical etching of CiO_x was detected by SEM, EDS and AFM. Making the method questionable for etching applications, there is substantial substrate damage associated with the etching. The SEM images indicate strongly inhomogeneous material removal, apparently initiated and propagated from specific but unidentified sites. In the experiments involving the Au/Pd film, there was phase separation of Au and Pd, and dewetting to form metallic islands. AFM data show that the etched holes were as deep as 200 nm, confirming relatively rapid etching of the Si substrate after the top layer of Cr oxide was removed.     

Substrate effect on mechanical relaxation of polystyrene in ultrathin films

Mechanical relaxation behavior in ultrathin polystyrene (PS) films supported on silicon oxide (SiO_x) and gold (Au) substrates has been studied by dynamic viscoelastic measurement. Based on the method, effects of free surface and substrate interface on the segmental dynamics were discussed. In the case of thin PS films with a thickness of approximately 200 nm, α_a-relaxation process corresponding to the segmental motion did not show any deviation from the bulk behavior. In contrast, for the films thinner than about 50 nm, the relaxation time distribution for the α_a-process became broader, probably due to a mobility gradient in the surface and interfacial regions. When we sandwiched an ultrathin PS film between SiO_x layers, another relaxation process, in addition to the original α_a-process, appeared at a higher temperature side that we assigned to the interfacial α_a-relaxation process. However, this was never seen for an ultrathin PS film between Au layers, implying that restriction from the substrate interface might be weak in this case.     

Dewetting process of Au films on SiO2 nanowires: Activation energy evaluation

SiO₂ nanowires gain scientific and technological interest in application fields ranging from nano-electronics, optics and photonics to bio-sensing. Furthermore, the SiO₂ nanowires chemical and physical properties, and so their performances in devices, can be enhanced if decorated by metal nanoparticles (such Au) due to local plasmonic effects.In the present paper, we propose a simple, low-cost and high-throughput three-steps methodology for the mass-production of Au nanoparticles coated SiO₂ nanowires. It is based on (1) production of the SiO₂ nanowires on Si surface by solid state reaction of an Au film with the Si substrate at high temperature; (2) sputtering deposition of Au on the SiO₂ nanowires to obtain the nanowires coated by an Au film; and (3) furnace annealing processes to induce the Au film dewetting on the SiO₂ nanowires surface. Using scanning electron microscopy analyses, we followed the change of the Au nanoparticles mean versus the annealing time extracting values for the characteristic activation energy of the dewetting process of the Au film on the SiO₂ nanowires surface. Such a study can allow the tuning of the nanowires/nanoparticles sizes for desired technological applications.