Two-dimensional Thin Films Research Articles

Accurate determination of two-dimensional thin film thickness in spectroscopic imaging reflectometer using color camera and tunable aperture

Abstract A new method is introduced to ensure accurate reconstruction of two-dimensional thin film thickness in a spectroscopic imaging reflectometer. Its simple configuration is an advantage of a system that employs color camera as a detector, however, it has the problem of ambiguity in determination of the thin film thickness. To secure reliable measurement performance, multi-reflectance method is applied. The angle of incidence can be changed by adjusting the diameter of the tunable aperture. After integrating multiple reflectance signals acquired at different angles of incidence, one combined signal enables exact determination of thin film thickness.

Strain Induced Metal–Insulator Transition of Magnetic SrRuO3 Single Layer in SrRuO3/SrTiO3 Superlattice

Ferromagnetic phase in a two-dimensional system plays an important role not only in applications but also in studies of phase transition theory. Among numerous ferromagnetic materials, Sr Ru O 3 is famous for its half-metallicity, itinerant ferromagnetism and non-Fermi liquid metalicity. Single layer Sr Ru O 3 in Sr Ru O 3 / Sr Ti O 3 (SRO/STO) superlattice has been predicted as a two-dimensional half-metallic ferromagnetic system based on density functional theory (DFT). However, experiments show that metal–insulator transition associated with ferro–antiferromagnetism (FM–AFM) transition occurs when the thickness of SRO is less than 4 u.c. Combining DFT calculations with Monte Carlo simulations, we demonstrate in this work that the bulk ferromagnetic metallicity can be realized in single layer SRO in SRO/STO superlattice by manipulating the strain effect to trigger the metal–insulator transition, achieving two-dimensional (2D) half-metallic SRO thin film beyond the experimental observation of AFM insulator.Our results pave a new route to fulfill the ultrathin spin-polarized-2D electron gas (SP-2DEG).

Growth of two-dimensional WS2 thin films by pulsed laser deposition technique

Tungsten disulfide (WS2) is a promising material for potential applications in next-generation 2D-electronics. However, for such applications large-area films will be required. Fabrication techniques involving exfoliation give films of high crystal quality but suffer from the problem of extremely small size of the films which makes it very challenging to realize real devices for industrial applications. In this study, we are reporting the growth of large area two-dimensional (2D) films of WS2 using pulsed laser deposition technique. A polycrystalline WS2 target was ablated using a pulsed KrF excimer laser (wavelength: 248 nm; pulse width: 25 ns). The ablated material was deposited over a c-axis aligned single crystal sapphire substrate maintained at 500°C in an ambient vacuum of 10−5 Torr. Thickness of the resulting films was controlled by controlling the number of incident laser pulses. Detailed characterization of the films was carried out using several state-of-the-art techniques including atomic force microscopy, Raman spectroscopy, and ultraviolet–visible spectroscopy. Relationship between the number of monolayers in the WS2 films and the spacing between the A1g(Γ) and E2g1 (Γ) Raman peaks (Δf) was determined. Effect of post annealing on the optical properties of the films was also investigated.

Study of two-dimensional boundary layer thin film fluid flow with variable thermo-physical properties in three dimensions space

The conversion of study in two dimensional (x, y) medium into three dimensions space (x, y, z) of a magnetohydrodynamic mixed convective heat and mass transfer boundary layer flow of a thin film second-grade fluid with temperature dependent viscosity and thermal conductivity in the presence of thermal radiation and viscous dissipation past a stretching sheet is analyzed. The occurrence of Hall current in two dimensional (x, y) medium produces a force in z-direction which generates a cross flow in that direction and so the motion is made in three dimensions space (x, y, z). Similarity transformations are used and the transformed system of equations of the problem has been solved by utilizing homotopy analysis method. The salient impacts of the emerging parameters on velocities, temperature and concentration fields have been displayed graphically and illustrated.

Confined Synthesis of Two-Dimensional Covalent Organic Framework Thin Films within Superspreading Water Layer.

The confined synthesis of two-dimensional covalent organic framework (2D COF) thin films was developed by using thin superspreading water on the hydrogel immersed under oil as reactor. Through loading two monomers into oil and hydrogel, respectively, COF thin films are synthesized at the oil/water/hydrogel interface. This strategy provides a new way for synthesis of freestanding 2D COF thin films. Detailed characterizations of the COF thin films reveal homogeneous topography, large area, controllable thickness from 4 to 150 nm, and crystallinity with certain orientation. Young's modulus of COF film is measured by AFM indentation as 25.9 ± 0.6 GPa, showing good mechanical properties. On the basis of the freestanding COF films, a nanofilter membrane and photoelectrochemical sensors for Ru3+ were successfully developed. Moreover, the strategy was extended to the synthesis of crystalline zeolitic imidazolate framework-8 thin film, which exhibited high application potential.

Growth of Copper Oxide Thin Films Deposited by Ultrasonic-Assisted Spray Pyrolysis Deposition Method

Copper oxide thin films are deposited using an ultrasonic-assisted spray pyrolysis deposition (SPD) system. To investigate the effect of substrate temperature and incorporation of a chelating agent on the growth of copper oxide thin films, the structural and optical properites of the copper oxide thin films are analyzed by X-ray diffraction (XRD), field-emssion scanning electron microscopy (FE-SEM), and UV-Vis spectrophotometry. At a temperature of less than 350 °C, threedimensional structures consisting of cube-shaped Cu2O are formed, while spherical small particles of the CuO phase are formed at a temperature higher than 400 °C due to a Volmer-Weber growth mode on the silicon substrate. As a chelating agent was added to the source solutions, two-dimensional Cu2O thin films are preferentially deposited at a temperature less than 300 °C, and the CuO thin film is formed even at a temperature less than 350 °C. Therefore the structure and crystalline phase of the copper oxide is shown to be controllable.

Dimension effect on the in-depth phase transition of MnAs/GaAs(001): From a thin film to a nanorod

We explore the dimension effect on the in-depth phase transition behavior of MnAs thin films on GaAs substrate by investigating the in-depth strain distribution with experiments and simulations. The strain distribution along the film growth direction is verified to be changed when a two-dimensional thin film is thinned into a one-dimensional nanorod as a cross-section transmission electron microscopy (TEM) specimen, thus resulting in a significantly different transition behavior. Our work brings more insights into the transition mechanism of MnAs thin films and illustrates a noteworthy issue in TEM studies of the phase transition at the interface of epitaxial thin films.

Measurement of two-dimensional thickness of micro-patterned thin film based on image restoration in a spectroscopic imaging reflectometer.

In this paper, we introduce a method to overcome the limitation of thickness measurement of a micro-patterned thin film. A spectroscopic imaging reflectometer system that consists of an acousto-optic tunable filter, a charge-coupled-device camera, and a high-magnitude objective lens was proposed, and a stack of multispectral images was generated. To secure improved accuracy and lateral resolution in the reconstruction of a two-dimensional thin film thickness, prior to the analysis of spectral reflectance profiles from each pixel of multispectral images, the image restoration based on an iterative deconvolution algorithm was applied to compensate for image degradation caused by blurring.

Synthesis of Sub-10 nm Two-Dimensional Covalent Organic Thin Film with Sharp Molecular Sieving Nanofiltration.

We demonstrated here a novel and facile synthesis of two-dimensional (2D) covalent organic thin film with pore size around 1.5 nm using a planar, amphiphilic and substituted heptacyclic truxene based triamine and a simple dialdehyde as building blocks by dynamic imine bond formation at the air/water interface using Langmuir-Blodgett (LB) method. Optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM), all unanimously showed the formation of large, molecularly thin and free-standing membrane that can be easily transferred on different substrate surfaces. The 2D membrane supported on a porous polysulfone showed a rejection rate of 64 and 71% for NaCl and MgSO4, respectively, and a clear molecular sieving at molecular size around 1.3 nm, which demonstrated a great potential in the application of pretreatment of seawater desalination and separation of organic molecules.

Critical current enhancement driven by suppression of superconducting fluctuation in ion-gated ultrathin FeSe

The framework of phase transition, such as superconducting transition, occasionally depends on the dimensionality of materials. Superconductivity is often weakened in the experimental conditions of two-dimensional thin films due to the fragile superconducting state against defects and interfacial effects. In contrast to this general trend, superconductivity in the thin limit of FeSe exhibits an opposite trend, such as an increase in critical temperature (Tc) and the superconducting gap exceeding the bulk values; however, the dominant mechanism is still under debate. Here, we measured thickness-dependent electrical transport properties of the ion-gated FeSe thin films to evaluate the superconducting critical current (Ic) in the ultrathin FeSe. Upon systematically decreasing the FeSe thickness by the electrochemical etching technique in the Hall bar-shaped electric double-layer transistors, we observed a dramatic enhancement of Ic reaching about 10 mA and corresponding to about 107 A cm−2 in the thinnest condition. By analyzing the transition behavior, we clarify that the suppressed superconducting fluctuation is one of the origins of the large Ic in the ion-gated ultrathin FeSe films. These results indicate the existence of a robust superconducting state possibly with dense Cooper pairs at the thin limit of FeSe.

Sodium citrate complexing agent-dependent growth of n- and p-type CdTe thin films for applications in CdTe/CdS based photovoltaic devices

A uniform and compact CdTe thin film is successfully electrodeposited on Ni flexible sheets. The influence of the concentration of sodium citrate in the growth solution on the morphological, compositional and electrical conductive properties of CdTe films is investigated. The results show that sodium citrate plays a significant role in directing the morphological evolution of CdTe from three-dimensional rod-shaped array to two-dimensional thin film. The composition of the film is changed from Te-rich to Cd-rich, and the conversion from p-type conduction to n-type in CdTe films is also achieved with the increase of sodium citrate concentration. When the concentration of sodium citrate is 0.08 mol L−1, the composition of CdTe film is near stoichiometric and the amount of excess Te is small. Furthermore, the effect of sodium citrate on the photoelectric conversion properties of CdTe-based solar cells is also analyzed. The fabricated solar cell based on the CdTe film prepared using 0.08 mol L−1 sodium citrate shows a short-circuit current density of 10.46 mA cm−2 and an open-circuit voltage of 0.53 V, yielding a better energy conversion efficiency of 2.73% with a fill factor of 49.28%. The better energy conversion efficiency is 2.1 and 1.6 times higher than that of the devices based on CdTe films prepared using 0.07 and 0.09 mol L−1 sodium citrate, respectively. This study indicates that the as-prepared CdTe thin film has a great potential for application in thin film solar cells, and the sodium citrate-assisted synthesis of CdTe film suggests a promising technique for large-scale fabrication of flexible solar cells.

Wide Spectral Photoresponse of Layered Platinum Diselenide-Based Photodiodes.

Platinum diselenide (PtSe2) is a group-10 transition metal dichalcogenide (TMD) that has unique electronic properties, in particular a semimetal-to-semiconductor transition when going from bulk to monolayer form. We report on vertical hybrid Schottky barrier diodes (SBDs) of two-dimensional (2D) PtSe2 thin films on crystalline n-type silicon. The diodes have been fabricated by transferring large-scale layered PtSe2 films, synthesized by thermally assisted conversion of predeposited Pt films at back-end-of-the-line CMOS compatible temperatures, onto SiO2/Si substrates. The diodes exhibit obvious rectifying behavior with a photoresponse under illumination. Spectral response analysis reveals a maximum responsivity of 490 mA/W at photon energies above the Si bandgap and relatively weak responsivity, in the range of 0.1-1.5 mA/W, at photon energies below the Si bandgap. In particular, the photoresponsivity of PtSe2 in infrared allows PtSe2 to be utilized as an absorber of infrared light with tunable sensitivity. The results of our study indicate that PtSe2 is a promising option for the development of infrared absorbers and detectors for optoelectronics applications with low-temperature processing conditions.

Microstructure and optical response optimization of Ge/Si quantum dots transformed from the sputtering-grown Ge thin film by manipulating the thermal annealing

A series of zero-dimensional Ge/Si quantum dots (QDs) samples are fabricated by inducing the transformation from the two-dimensional Ge thin film, which is grown by the traditional direct current (DC) magnetron sputtering, via regulating the annealing process. The QD density increases sharply after the post rapid thermal annealing (PRTA). The observations of atomic force microscopy (AFM) and Raman spectroscopy suggest that the good morphology of Ge QDs results from an appropriate thermodynamics and kinetics surrounding shaped by the cooperative interaction of the Ge–Si lattice mismatch, the film’s surface temperature, and the difference in thermal expansion coefficients between Ge and Si. The photoluminescence (PL) peaks of Ge QDs are detected in monolayer Ge QDs with ultrahigh density at 17 K. The Metal-Ge/Si QDs-Metal (MGM) photodetector fabricated from the ultrahigh-density QDs sample exhibits a relatively high current gain, absolute photoelectric responsivity, and internal quantum efficiency (IQE). Our results demonstrate that the high-quality Ge QDs with strong light absorption and quantum confinement effect can be realized by modulating DC magnetron sputtering and the PRTA process. This paves the way for realizing silicon-based optoelectronic devices with high performance by the traditional, relatively low-cost, and large-scale production nanomaterial fabricating method.

Upregulation of osteogenesis of mesenchymal stem cells with virus-based thin films.

A major aim of tissue engineering is to develop biomimetic scaffolding materials that can guide the proliferation, self-renewal and differentiation of multipotent stem cells into specific lineages. Cellular functions can be controlled by the interactions between cells and biomaterials. Therefore, the surface chemistry and topography of support materials play a pivotal role in modulating cell behaviors at many stages of cell growth and development. Due to their highly ordered structure and programmable surface chemistries, which provide unique topography as biomaterials, viral nanoparticles have been utilized as building blocks for targeted cell growth and differentiation. This review article discusses the fabrication of two-dimensional virus-based thin film on substrates and highlights the study of the effect of chemical and physical cues introduced by plant virus nanoparticle thin films on the promotion of osteogenic differentiation of BMSCs.

Analytical validation of a $2+1$ dimensional continuum model for epitaxial growth with elastic substrate

An analiyical validation is obtained for the evolution equation ht = ∆[F−1(−aEF(h))− r/h2 −∆h], introduced in [18] by W.T. Tekalign and B.J. Spencer to describe the heteroepitaxial growth of a two-dimensional thin film on an elastic substrate. In the expression above, h denotes the surface height of the film, F is the Fourier transform, and a, E, r are positive material constants. Existence, uniqueness, and Lipschitz regularity in time for weak solutions are proved, under suitable assumptions on the initial datum. Introduction The epitaxial deposition of a thin film on a relatively thick substrate has gained much interest in the recent years due to its applications to semiconductor electronics and quantum dots. Roughly speaking, the morphology of the film is known to be the result of a competition between the elastic energy associated to the mismatch between film and substrate, and the surface mass transport due to the film deposition. An extensive mathematical analysis of the mechanism associated to epitaxial film growth has been carried out in [2, 3, 10, 11, 15, 16] in the context of plane linear elasticity, and regularity results have been established for volume-constrained minimizers. Short time existence for a surface diffusion type geometric evolution equation keeping into account elasticity has first been analyzed in [12] in a two-dimensional setting (see also [17]). The previous result has been recently extended in [13] to the three-dimensional case. The central aim of this work is to study existence and Lipschitz regularity in time of weak solutions to the 2+1 dimensional evolution equation ht = ∆[F−1(−aEF(h))− r/h2 −∆h] (0.1) derived by W.T. Tekalign and B.J. Spencer in [18], where h denotes the surface height of the film and F is the Fourier transform. The quantities a, E, r are positive material constants. To be precise, a (resp. r) is the wavenumber (resp. wetting coefficient) associated to the equation, and E is defined as E := 2μ F (1 + ν )(1− ν) (1− νF )μS , where μ and ν (resp. μ and ν) are the elastic shear modulus and the Poisson’s 2010 Mathematics Subject Classification. 35K55, 35K67, 44A15, 74K35.

Theoretical Discovery of a Superconducting Two-Dimensional Metal-Organic Framework.

Superconductivity is a fascinating quantum phenomenon characterized by zero electrical resistance and the Meissner effect. To date, several distinct families of superconductors (SCs) have been discovered. These include three-dimensional (3D) bulk SCs in both inorganic and organic materials as well as two-dimensional (2D) thin film SCs but only in inorganic materials. Here we predict superconductivity in 2D and 3D organic metal-organic frameworks by using first-principles calculations. We show that the highly conductive and recently synthesized Cu-benzenehexathial (BHT) is a Bardeen-Cooper-Schrieffer SC. Remarkably, the monolayer Cu-BHT has a critical temperature (Tc) of 4.43 K, while Tc of bulk Cu-BHT is 1.58 K. Different from the enhanced Tc in 2D inorganic SCs which is induced by interfacial effects, the Tc enhancement in this 2D organic SC is revealed to be the out-of-plane soft-mode vibrations, analogous to surface mode enhancement originally proposed by Ginzburg. Our findings not only shed new light on better understanding 2D superconductivity but also open a new direction to search for SCs by interface engineering with organic materials.

Te Monolayer-Driven Spontaneous van der Waals Epitaxy of Two-dimensional Pnictogen Chalcogenide Film on Sapphire.

Demands on high-quality layer structured two-dimensional (2D) thin films such as pnictogen chalcogenides and transition metal dichalcogenides are growing due to the findings of exotic physical properties and potentials for device applications. However, the difficulties in controlling epitaxial growth and the unclear understanding of van der Waals epitaxy (vdWE) for a 2D chalcogenide film on a three-dimensional (3D) substrate have been major obstacles for the further advances of 2D materials. Here, we exploit the spontaneous vdWE of a high-quality 2D chalcogenide (Bi0.5Sb1.5Te3) film by the chalcogen-driven surface reconstruction of a conventional 3D sapphire substrate. It is verified that the in situ formation of a pseudomorphic Te atomic monolayer on the surface of sapphire, which results in a dangling bond-free surface, allows the spontaneous vdWE of 2D chalcogenide film. Since this route uses the natural surface reconstruction of sapphire with chalcogen under vacuum condition, it can be scalable and easily utilized for the developments of various 2D chalcogenide vdWE films through conventional thin-film fabrication technologies.

Wafer-scale two-dimensional ferromagnetic Fe3GeTe2 thin films grown by molecular beam epitaxy

Recently, layered two-dimensional ferromagnetic materials (2D FMs) have attracted a great deal of interest for developing low-dimensional magnetic and spintronic devices. Mechanically exfoliated 2D FMs were discovered to possess ferromagnetism down to monolayer. It is therefore of great importance to investigate the distinct magnetic properties at low dimensionality. Here, we report the wafer-scale growth of 2D ferromagnetic thin films of Fe3GeTe2 via molecular beam epitaxy, and their exotic magnetic properties can be manipulated via the Fe composition and the interface coupling with antiferromagnetic MnTe. A 2D layer-by-layer growth mode has been achieved by in situ reflection high-energy electron diffraction oscillations, yielding a well-defined interlayer distance of 0.82 nm along {002} surface. The magnetic easy axis is oriented along c-axis with a Curie temperature of 216.4 K. Remarkably, the Curie temperature can be enhanced when raising the Fe composition. Upon coupling with MnTe, the coercive field dramatically increases 50% from 0.65 to 0.94 Tesla. The large-scale layer-by-layer growth and controllable magnetic properties make Fe3GeTe2 a promising candidate for spintronic applications. It also opens up unprecedented opportunities to explore rich physics when coupled with other 2D superconductors and topological matters.

Two-Dimensional Semiconductors Thin Films of MoS2 and MoSe2 on Ag(111)

In the recent years, technological development in the fields spanning from nanoelectronics to new solar energy materials, requires highly controlled nanostructured surfaces, ultra-thin films and 2D structured materials. The Electrochemical Atomic Layer Deposition (E-ALD) is one of these techniques that allows us to produce thin films of semiconductor. Exploiting Surface Limited Reactions (SLR), it enables the deposition of highly ordered ultra-thin films from diluted aqueous solutions and at room temperature and pressure. Underpotential deposition (UPD) is a type of electrochemical SLR, where an atomic layer of a first element is deposited on a second, at a potential prior to (under) that needed to deposit the first element on itself, so that the resulting deposit is generally limited to an atomic layer. It occurs when the depositing element is able to somehow interact with the substrate, so that the deposition of the layer in direct contact with the substrate occurs at a potential preceding bulk deposition, that is, the deposition of the element on itself. MoS2 and MoSe2 are important transition metal dichalcogenides (TMDs). E-ALD method was used to obtain semiconductor compounds in the form of thin films. UPD anodic electrodeposition of Na2S and Na2SeO3 on crystalline Ag[111] electrode are well-known. The research move to discover the optimal conditions to deposit Mo on Ag/S and on Ag/Se from a solution of MoO4 2- in alkaline buffer. This study has been carried by means of voltammetric techniques, XPS and AFM characterizations of the deposit.

Spray Pyrolysis Deposition of Zinc Oxide Thin Films by ZnO Buffer Layer

We investigated the effect of ZnO buffer layer on the formation of ZnO thin film by ultrasonic assisted spray pyrolysis deposition. ZnO buffer layer was formed by wet solution method, which was repeated several times. Structural and optical properties of the ZnO thin films deposited on the ZnO buffer layers with various cycles and at various temperatures were investigated by field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence spectrum analysis. The structural investigations showed that three-dimensional island shaped ZnO was formed on the bare Si substrate without buffer layers, while two-dimensional ZnO thin film was deposited on the ZnO buffer layers. In addition, structural and optical investigations showed that the crystalline quality of ZnO thin film was improved by introducing the buffer layers. This improvement was attributed to the modulation of the surface energy of the Si surface by the ZnO buffer layer, which finally resulted in a modification of the growth mode from three to two-dimensional.