Two-dimensional Thin Films Research Articles

Triple junction controlled grain growth in two-dimensional polycrystals and thin films: Self-similar growth laws and grain size distributions

Triple junction controlled grain growth in two-dimensional nanocrystalline polycrystals is modeled by attributing to each structural feature of a polygonal grain a finite mobility. By considering grain growth as a dissipative process that is driven by the reduction of the Gibbs free interface energy, a general grain evolution equation is derived that separates into two types of possible self-similar growth kinetics. For the case of pure triple junction drag the influence of finite triple junction mobilities on metrical and topological properties is studied. Analytical expressions of the self-similar grain size distribution are derived, which compare very well with results from the modified Monte Carlo Potts model and front-tracking vertex dynamic simulations, taking into account size effects in triple junction limited grain growth. In addition, the analytical grain size distributions are used for a theoretical description of experimental data obtained in nanocrystalline thin films upon annealing.

Investigations on the morphological evolution of zinc oxide nanostructures and their optical properties

The morphological evolution of ZnO nanostructures grown under various argon sputtering pressures and substrate temperatures has been systematically investigated. A surface phase diagram depicting compact two-dimensional thin films to various types of one-dimensional structures as a function of pressure and temperature is constructed. Structural studies of ZnO nanostructures reveal the preferential orientation along the c-axis of a hexagonal wurtzite crystal structure with a highly crystalline nature. However, the as-grown ZnO nanostructures slightly experience compressive strain, which increases with the argon sputtering pressure. Exciton recombination and point defects have been extensively analyzed by temperature-dependent photoluminescence spectroscopy. The observation of donor-bound exciton emission and its replicas reveals the high optical quality of the nanostructures. Nevertheless, the observed visible emission indicates the presence of point defects which significantly depend on the deposition pressure. The characteristic visible emission bands at 2.28 and 2.02 eV provide strong evidence for the existence of oxygen vacancies and interstitial sites in ZnO nanostructures irrespective of growth temperature and pressure.

Numerical modelling of the transient heat transport in 2D silicon thin film using the interval lattice Boltzmann method

Journal of Applied Mathematics and Computational Mechanics, Prace Naukowe Instytutu Matematyki i Informatyki, Politechnika Częstochowska, Scientific Research of the Institute of Mathematics and Computer Science, Czestochowa University of Technology

Biomineralization-inspired approach to the development of hybrid materials: preparation of patterned polymer/strontium carbonate thin films using thermoresponsive polymer brush matrices

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAm) brushes have been synthesized and employed as the crystallization matrices for the development of polymer/strontium carbonate hybrid materials. The use of PNIPAm brush matrices results in the formation of patterned polymer/strontium carbonate hybrid thin films in the presence of poly(acrylic acid) additives. Moreover, the conformational change in the thermoresponsive PNIPAm brush matrices induces the change in the surface morphologies of the obtained polymer/SrCO3 hybrid thin films. The strategy of using stimuli-responsive polymer brush matrices is expected to be a promising method for controlling the surface morphology of two-dimensional hybrid thin film materials. The use of poly(N-isopropylacrylamide) (PNIPAm) brush matrices in the presence of poly(acrylic acid) additives at temperatures above and below the lower critical solution temperature (LCST) induced the development of PNIPAm/strontium carbonate hybrid thin films with distinctly different surface patterns.

All two-dimensional, flexible, transparent, and thinnest thin film transistor.

In this article, we report only 10 atomic layer thick, high mobility, transparent thin film transistors (TFTs) with ambipolar device characteristics fabricated on both a conventional silicon platform as well as on a flexible substrate. Monolayer graphene was used as metal electrodes, 3-4 atomic layers of h-BN were used as the gate dielectric, and finally bilayers of WSe2 were used as the semiconducting channel material for the TFTs. The field effect carrier mobility was extracted to be 45 cm(2)/(V s), which exceeds the mobility values of state of the art amorphous silicon based TFTs by ∼100 times. The active device stack of WSe2-hBN-graphene was found to be more than 88% transparent over the entire visible spectrum and the device characteristics were unaltered for in-plane mechanical strain of up to 2%. The device demonstrated remarkable temperature stability over 77-400 K. Low contact resistance value of 1.4 kΩ-μm, subthreshold slope of 90 mv/decade, current ON-OFF ratio of 10(7), and presence of both electron and hole conduction were observed in our all two-dimensional (2D) TFTs, which are extremely desirable but rarely reported characteristics of most of the organic and inorganic TFTs. To the best of our knowledge, this is the first report of all 2D transparent TFT fabricated on flexible substrate along with the highest mobility and current ON-OFF ratio.

Influence of heat source size and film thickness on phonon transport in a two-dimensional thin film

Abstract. Phonon transport in a two-dimensional thin silicon film is considered and the effect of heat source size and the film thickness on the transport characteristics is examined. Frequency dependent Boltzmann equation is incorporated in the analysis to account for the contribution of the ballistic phonons to the energy transport. Equivalent equilibrium temperature is introduced to assess the thermal resistance during the phonon transport in the film. The numerical scheme with the appropriate boundary conditions is used to predict the transport properties, including the effective thermal conductivity, of the thin film. It is found that the heat source size and the film thickness influence the thermal resistance of the film almost equally. The ballistic phonons reduce the film thermal resistance while suppressing the effective thermal conductivity in the thin film.

Static and low frequency noise characterization of N-type random network of carbon nanotubes thin film transistors

Static and low frequency noise (LFN) characterizations in two-dimensional (2D) N-type random network thin film transistors (RN-TFTs) based on single-walled carbon nanotubes were presented. For the electrical parameter extraction, the Y-function method was used to suppress the series resistance (Rsd) influence. The gate-to-channel capacitance (Cgc) was directly measured by the split capacitance-to-voltage method and compared to 2D metal-plate capacitance model (C2D). In addition, to account for the percolation-dominated 2D RN-TFTs, a numerical percolation simulation was performed. LFN measurements were also carried out and the results were well interpreted by the carrier number and correlated mobility fluctuation model. Finally, one-dimensional (1D) cylindrical analytical capacitance based model (C1D) was suggested and applied to provide better consistency between all electrical parameters based on experimental and simulation results.

Mixing in thermal convection of very thin free-standing films

Thin liquid films serve as the paradigms of atmospheric convection, thermal convection in the Earth's mantle or turbulence in magnetohydrodynamics, thereby connecting with typical systems exhibiting turbulent mixing. In addition, recent research on colloids, interfaces and nanofluids led to advances in the development of micro-mixers (lab-on-a-chip devices). By the thermal forcing of a film, one can reach Rayleigh numbers in the turbulent regime, such that the experiment may serve as a prototype of a mixer on the basis of thermally induced turbulence in free-standing thin liquid films with thicknesses in the nanometer range. Here, the specific experimental results of a setup with a focus on the mixing statistics of a thermally driven two-dimensional thin film are presented. Our setup allows us to capture thin film interference patterns under controlled surface and atmospheric conditions. The convection is realized by placing a cooled copper rod in the center of the film. The temperature gradient between the rod and the atmosphere results in a density gradient in the liquid film, so that the varying buoyancy induces turbulent motion. The flow field is characterized by a newly developed algorithm—color imaging velocimetry (CIV). This analysis determines not only the velocity, but also the full deformation tensor in the Lagrangian frame. On the basis of these insights, the flow in the experiment was investigated with respect to its mixing properties: we calculated the mixing efficiency and entropy of the flow scheme to sufficiently high accuracy.

ChemInform Abstract: Three Dimensional Macroporous Architectures and Aerogels Built of Carbon Nanotubes and/or Graphene: Synthesis and Applications

AbstractReview: 211 refs.

Smallest Metallic Nanorods Using Physical Vapor Deposition

Physical vapor deposition provides a controllable means of growing two-dimensional metallic thin films and one-dimensional metallic nanorods. While theories exist for the growth of metallic thin films, their counterpart for the growth of metallic nanorods is absent. Because of this absence, the lower limit of the nanorod diameter is theoretically unknown; consequently the experimental pursuit of the smallest nanorods has no clear target. This Letter reports a closed-form theory that defines the diameter of the smallest metallic nanorods using physical vapor deposition. Further, the authors verify the theory using lattice kinetic MonteCarlo simulations and validate the theory using published experimental data. Finally, the authors carry out a series of theory-guided experiments to grow well-separated metallic nanorods of ∼10 nm in diameter, which are the smallest ever reported using physical vapor deposition.

NUMERICAL GRADIENT FLOW DISCRETIZATION OF VISCOUS THIN FILMS ON CURVED GEOMETRIES

The evolution of a viscous thin film on a curved geometry is numerically approximated based on the natural time discretization of the underlying gradient flow. This discretization leads to a variational problem to be solved at each time step, which reflects the balance between the decay of the free (gravitational and surface) energy and the viscous dissipation. Both dissipation and energy are derived from a lubrication approximation for a small ratio between the characteristic film height and the characteristic length scale of the surface. The dissipation is formulated in terms of a corresponding flux field, whereas the energy primarily depends on the fluid volume per unit surface, which is a conserved quantity. These two degrees of freedom are coupled by the underlying transport equation. Hence, one is naturally led to a PDE-constrained optimization problem, where the variational time stepping problem has to be solved under the constraint described by the transport equation. For the space discretization a discrete exterior calculus approach is investigated. Various applications demonstrate the qualitative and quantitative behavior of one- and two-dimensional thin films on curved geometries.

G132 メソポーラスシリカSBA-15薄膜の細孔構造制御(OS-7:マイクロ・ナノ熱工学(3))

In this study, two-dimensional hexagonal mesoporous silica thin films of SBA-15 were synthesized via dip-coating using an evaporation-induced self-assembly process. The effect of the withdrawal speed on the thickness of the films, one-dimensional pore alignments and two-dimensional hexagonal pore arrays was elucidated. The film thickness increases with increasing withdrawal speed and also increases upon decreasing the withdrawal speed to ultraslow region where capillary rise of the solution occurs. Detailed analyses of FE-SEM and TEM images and XRD patterns of the synthesized thin films clarified that the pore sizes and interplanar spacing also depend on the withdrawal speed; this suggests that the film thickness and hexagonal pore-array dimensions can be controlled without changing the physicochemical properties of the silica precursor solutions or the sizes of the template molecules.

Three dimensional macroporous architectures and aerogels built of carbon nanotubes and/or graphene: synthesis and applications

Carbon nanotubes and graphene are some of the most intensively explored carbon allotropes in materials science. This interest mainly resides in their unique properties with electrical conductivities as high as 10(4) S cm(-1), thermal conductivities as high as 5000 W m(-1) K and superior mechanical properties with elastic moduli on the order of 1 TPa for both of them. The possibility to translate the individual properties of these monodimensional (e.g. carbon nanotubes) and bidimensional (e.g. graphene) building units into two-dimensional free-standing thick and thin films has paved the way for using these allotropes in a number of applications (including photocatalysis, electrochemistry, electronics and optoelectronics, among others) as well as for the preparation of biological and chemical sensors. More recently and while recognizing the tremendous interest of these two-dimensional structures, researchers are noticing that the performance of certain devices can experience a significant enhancement by the use of three-dimensional architectures and/or aerogels because of the increase of active material per projected area. This is obviously the case as long as the nanometre-sized building units remain accessible so that the concept of hierarchical three-dimensional organization is critical to guarantee the mass transport and, as consequence, performance enhancement. Thus, this review aims to describe the different synthetic processes used for preparation of these three-dimensional architectures and/or aerogels containing either any or both allotropes, and the different fields of application in which the particular structure of these materials provided a significant enhancement in the efficacy as compared to their two-dimensional analogues or even opened the path to novel applications. The unprecedented compilation of information from both CNT- and graphene-based three-dimensional architectures and/or aerogels in a single revision is also of interest because it allows a straightforward comparison between the particular features provided by each allotrope.

Evolution of elastic thin films with curvature regularization via minimizing movements

The evolution equation with curvature regularization that models the motion of a two-dimensional thin film by evaporation-condensation on a rigid substrate is considered. The film is strained due to the mismatch between the crystalline lattices of the two materials. Here, short time existence, uniqueness and regularity of the solution are established using De Giorgi’s minimizing movements to exploit the $$L^{2}$$ -gradient flow structure of the equation. This seems to be the first analytical result for the evaporation-condensation case in the presence of elasticity.

Enhanced thermoelectric performance in three-dimensional superlattice of topological insulator thin films

We show that certain three-dimensional (3D) superlattice nanostructure based on Bi2Te3 topological insulator thin films has better thermoelectric performance than two-dimensional (2D) thin films. The 3D superlattice shows a predicted peak value of ZT of approximately 6 for gapped surface states at room temperature and retains a high figure of merit ZT of approximately 2.5 for gapless surface states. In contrast, 2D thin films with gapless surface states show no advantage over bulk Bi2Te3. The enhancement of the thermoelectric performance originates from a combination of the reduction of lattice thermal conductivity by phonon-interface scattering, the high mobility of the topologically protected surface states, the enhancement of Seebeck coefficient, and the reduction of electron thermal conductivity by energy filtering. Our study shows that the nanostructure design of topological insulators provides a possible new way of ZT enhancement.

Phonon transport in two-dimensional silicon thin film: influence of film width and boundary conditions on temperature distribution

Radiative phonon transport in two-dimensional silicon thin film is considered and equivalent equilibrium temperature is analyzed for different boundary conditions at the film faces. The influence of the film thickness on phonon transport is also examined and limiting film thickness for two-dimensional phonon transport is demonstrated. It is found that the two-dimensional phonon transport reduces to one-dimensional transport for the film width more than or equal to the twice of the thickness of the silicon film. Equivalent equilibrium temperature predicted, agrees well with the previous findings.

Electron dephasing in homogeneous and inhomogeneous indium tin oxide thin films

The electron dephasing processes in two-dimensional homogeneous and inhomogeneous indium tin oxide thin films have been investigated in a wide temperature range 0.3--90 K. We found that the small-energy-transfer electron-electron ($e$-$e$) scattering process dominated the dephasing from a few K to several tens K. At higher temperatures, a crossover to the large-energy-transfer $e$-$e$ scattering process was observed. Below about 1--2 K, the dephasing time $\tau_\varphi$ revealed a very weak temperature dependence, which intriguingly scaled approximately with the inverse of the electron diffusion constant $D$, i.e., $\tau_\varphi (T \approx 0.3 \, {\rm K}) \propto 1/D$. Theoretical implications of our results are discussed. The reason why the electron-phonon relaxation rate is negligibly weak in this low-carrier-concentration material is presented.

An asymmetric oligomer based on thienoacene for solution processed crystal organic thin-film transistors

A novel thienoacene-based conjugated oligomer, i.e.BTTT-T-C12, was designed and synthesized. Its highly asymmetric structural feature enables the preparation of two-dimensional single-crystalline thin films in millimetre size and ~100 nm thick by a solution processing method directly on the Si/SiO(2) substrate. Single crystal organic thin film transistors exhibit a mobility of 0.70 cm(2) V(-1) s(-1) and an on/off ratio of 5.7 × 10(4).

1/f Noise in Length Sorted Single-walled Carbon Nanotubes at Their Critical Percolation Conditions

ABSTRACTThe electrical noise characteristics of thin film random networks of single walled carbon nanotubes with lengths of 820nm, 210nm and 130nm, were evaluated in addition to mixed length and pure semiconducting single-walled carbon nanotube networks. This study represents one of the first experimental studies in which highly characterized length sorted single walled nanotubes networks have been investigated to isolate their contributions to 1/f noise. In this work we evaluate the noise power spectrum, in the low frequency range, for each of our type sorted samples and demonstrate the effect of nanotube type, length, dimensionality and critical percolation conditions in 1/f noise generating mechanisms. 1/f noise in two-dimensional (2-D) thin films of random network, homogeneous length sorted SWNTs at their percolation threshold in contrast to three dimensional (3-D) thin films of mixed length SWNT and purely semi-conducting SWNT thin films were investigated. We find that at their respective critical percolation thresholds, xc, length sorted SWNT networks exhibit atypical reduced noise amplitude (A) characteristics compared to their mixed length and semi-conducting nanotube counterparts.

Dewetting dynamics of silicon-on-insulator thin films

Using low-energy electron microscopy (LEEM), we have measured, in real time, the dewetting of single-crystal Si(001) thin films on amorphous silicon dioxide substrates, which transforms the two-dimensional (2D) thin film into three-dimensional (3D) compact Si nanocrystals. The dewetting scenario has been reported by Bussmann et al. [New J. Phys. 13, 043017 (2011)]. Analytic 2D and 3D models based on simple approximate geometries of the dewetting front have been developed to analyze LEEM measurements. They enable us to estimate the driving force for dewetting ${E}_{s}\ensuremath{\sim}14$ eV/nm${}^{2}$. Starting from a Si-film thickness dependent effective dewetting activation barrier, a single Si(001) surface self-diffusion energy of ${E}_{a}=2.0\ifmmode\pm\else\textpm\fi{}0.2$ eV is derived. First nanoisland-formation dynamics measurements are discussed. Finally, grazing incidence small-angle x-ray scattering (GISAXS) is used to characterize the structure and the morphology of the Si nanocrystals created by the dewetting process.