Browsing by Subject "Nanostructures"

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Aspects of metal and Si-based nanomaterials: synthesis, stability and properties
(2006) Elechiguerra Joven, José Luis; Yacaman, Miguel Jose
Metal and Si-based nanostructures have drawn increasing interest due to their potential uses in catalysis, biological sensors, and nanoelectronics among others. Therefore, in the present work, several nanostructures were produced, characterized and tested. In particular, the conventional synthesis of noble-metal nanostructures through the polyol method was modified by replacing poly-vinyl pyrrolidone PVP with poly-diallyl dimethyl ammonium chloride PDDA. As PDDA is a cationic polyelectrolyte, the initial strong electrostatic interaction between PDDA and the anionic metal precursors produce the formation of stable ion pairs, so the reactivity of the different species can be tailored and particles with different internal structure, i.e. crystallinity, can be produced.
A comprehensive study of 3D nano structures characteristics and novel devices
(2008-12) Zaman, Rownak Jyoti; Banerjee, Sanjay
Silicon based 3D fin structure is believed to be the potential future of current semiconductor technology. However, there are significant challenges still exist in realizing a manufacturable fin based process. In this work, we have studied the effects of hydrogen anneal on the structural and electrical characteristics of silicon fin based devices: tri-gate, finFET to name a few. H₂ anneal is shown to play a major role in structural integrity and manufacturability of 3D fin structure which is the most critical feature for these types of devices. Both the temperature and the pressure of H₂ anneal can result in significant alteration of fin height and shape as well as electrical characteristics. Optimum H₂ anneal is required in order to improve carrier mobility and device reliability as shown in this work. A new hard-mask based process was developed to retain H₂ anneal related benefit while eliminating detrimental effects such as reduction of device drive current due to fin height reduction. We have also demonstrated a novel 1T-1C pseudo Static Random Access Memory (1T-1C pseudo SRAM) memory cell using low cost conventional tri-gate process by utilizing selective H₂ anneal and other clever process techniques. TCAD-based simulation was also provided to show its competitive advantage over other types of static and dynamic memories in 45nm and beyond technologies. A high gain bipolar based on silicon fin process flow was proposed for the first time that can be used in BiCMOS technology suitable for low cost mixed signal and RF products. TCAD-based simulation results proved the concept with gain as high 100 for a NPN device using single additional mask. Overall, this work has shown that several novel process techniques and selective use of optimum H₂ anneal can lead to various high performance and low cost devices and memory cells those are much better than the devices using current conventional 3D fin based process techniques.
Electronic transport under strong optical radiation and quantum chaos in semiconductor nanostructures
(2003) Li, Wenjun; Reichl, L. E.
Experimental and theoretical investigation of thermal and thermoelectric transport in nanostructures
(2010-05) Moore, Arden Lot, 1982-; Shi, Li, Ph. D.; Ezekoye, Ofodike; Ferreira, Paulo; Howell, John; Tutuc, Emanuel
This work presents the development and application of analytical, numerical, and experimental methods for the study of thermal and electrical transport in nanoscale systems, with special emphasis on those materials and phenomena which can be important in thermoelectric and semiconductor device applications. Analytical solutions to the Boltzmann transport equation (BTE) using the relaxation time approximation (RTA) are presented and used to study the thermal and electrical transport properties of indium antimonide (InSb), indium arsenide (InAs), bismuth telluride (Bi₂Te₃), and chromium disilicide (CrSi₂) nanowires. Experimental results for the thermal conductivity of single layer graphene supported by SiO₂ were analyzed using an RTA-based model and compared to a full quantum mechanical numerical BTE solution which does not rely on the RTA. The ability of these models to explain the measurement results as well as differences between the two approaches are discussed. Alternatively, numerical solutions to the BTE may be obtained statistically through Monte Carlo simulation for complex geometries which may prove intractable for analytical methods. Following this approach, phonon transport in silicon (Si) sawtooth nanowires was studied, revealing that thermal conductivity suppression below the diffuse surface limit is possible. The experimental investigation of energy transport in nanostructures typically involved the use of microfabricated devices or non-contact optical methods. In this work, two such approaches were analyzed to ascertain their thermal behavior and overall accuracy as well as areas for possible improvement. A Raman spectroscopy-based measurement design for investigating the thermal properties of suspended and supported graphene was examined analytically. The resulting analysis provided a means of determining from measurement results the thermal interface conductance, thermal contact resistance, and thermal conductivity of the suspended and supported graphene regions. Previously, microfabricated devices of several different designs have been used to experimentally measure the thermal transport characteristics of nanostructures such as carbon nanotubes, nanowires, and thin films. To ascertain the accuracy and limitations of various microdevice designs and their associated conduction analyses, finite element models were constructed using ANSYS and measurements of samples of known thermal conductance were simulated. It was found that designs with the sample suspended were generally more accurate than those for which the sample is supported on a bridge whose conductance is measured separately. The effects of radiation loss to the environment of certain device designs were also studied, demonstrating the need for radiation shielding to be at temperatures close to that of the device substrate in order to accurately calibrate the resistance thermometers. Using a suspended microdevice like those analyzed using finite element analysis, the thermal conductivities of individual bismuth (Bi) nanowires were measured. The results were correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk Bi in the same crystal direction, the thermal conductivity of a single-crystal Bi nanowires of 232 nm diameter was found to be 3 - 6 times smaller than bulk between 100 K and 300 K. For polycrystalline Bi nanowires of 74 nm to 255 nm diameter the thermal conductivity was reduced by a factor of 18 - 78 over the same temperature range. Comparable thermal conductivity values were measured for polycrystalline nanowires of varying diameters, suggesting a grain boundary scattering mean free path for all heat carriers in the range of 15 - 40 nm which is smaller than the nanowire diameters. An RTA-based transport model for both charge carriers and phonons was developed which explains the thermal conductivity suppression in the single-crystal nanowire by considering diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 10¹⁹ cm⁻³. Using a similar experimental setup, the thermoelectric properties (Seebeck coefficient, electrical conductivity, and thermal conductivity) of higher manganese silicide (HMS) nanostructures were investigated. Bulk HMS is a passable high temperature thermoelectric material which possesses a complex crystal structure that could lead to very interesting and useful nanoscale transport properties. The thermal conductivities of HMS nanowires and nanoribbons were found to be reduced by 50 - 60 % compared to bulk values in the same crystal direction for both nanoribbons and nanowires. The measured Seebeck coefficient data was comparable or below that of bulk, suggesting unintentional doping of the samples either during growth or sample preparation. Difficulty in determining the amorphous oxide layer thickness for nanoribbons samples necessitated using the total, oxide-included cross section in the thermal and electrical conductivity calculation. This in turn led to the determined electrical conductivity values representing the lower bound on the actual electrical conductivity of the HMS core. From this approach, the measured electrical conductivity values were comparable or slightly below the lower end of bulk electrical conductivity values. This oxide thickness issue affects the determination of the HMS nanostructure thermoelectric figure of merit ZT as well, though the lower bound values obtained here were found to still be comparable to or slightly smaller than the expected bulk values in the same crystal direction. Analytical modeling also indicates higher doping than in bulk. Overall, HMS nanostructures appear to have the potential to demonstrate measurable size-induced ZT enhancement, especially if optimal doping and control over the crystallographic growth direction can be achieved. However, experimental methods to achieve reliable electrical contact to quality four-probe samples needs to be improved in order to fully investigate the thermoelectric potential of HMS nanostructures.
First-principles atomistic modeling for property prediction in silicon-based materials
(2010-12) Bondi, Robert James; Hwang, Gyeong S.; Mullins, C. B.; Ekerdt, John G.; Chelikowsky, James R.; Banerjee, Sanjay K.
The power of parallel supercomputing resources has progressed to the point where first-principles calculations involving systems up to 10³ atoms are feasible, allowing ab initio exploration of increasingly complex systems such as amorphous networks, nanostructures, and large defect clusters. Expansion of our fundamental understanding of modified Si-based materials is paramount, as these materials will likely flourish in the foreseeable cost-driven future in diverse micro- and nanotechnologies. Here, density-functional theory calculations within the generalized gradient approximation are applied to refine configurations of Si-based materials generated from Metropolis Monte Carlo simulations and study their resultant structural properties. Particular emphasis is given to the contributions of strain and disorder on the mechanical, optical, and electronic properties of modified Si-based materials in which aspects of compositional variation, phase, strain scheme, morphology, native defect incorporation, and quantum confinement are considered. The simulation strategies discussed are easily extendable to other semiconductor systems.
Gas transport properties of reverse selective nanocomposite materials
(2007-12) Matteucci, Scott Tyson, 1976-; Freeman, B. D., (Benny D.)
The effect of dispersing discreet periclase (magnesium oxide) or brookite (titanium oxide) nanoparticles into poly(1-trimethylsilyl-1-propyne) (i.e., a super glassy polymer) and 1,2-polybutadiene (i.e., a rubbery polymer) has been examined. Particle dispersion has been investigated using atomic force microscopy and transmission electron microscopy to determine particle/aggregate size and distribution. Titanium dioxide nanoparticles dispersed into aggregates on the order of nanometers, as did magnesium oxide in 1,2-polybutadiene. However, the magnesium oxide filled poly(1-trimethylsilyl-1-propyne) did not exhibit nanoparticle aggregates below approximately one micron in characteristic dimensions. Nanocomposite transport properties were studied, where permeability and solubility coefficients were determined for light gases with increasing pressure, and diffusion coefficients were calculated from the solution-diffusion model. The permeability of light gases in the heterogeneous films increased with increasing particle loading. Depending on particle loading, brookite filled nanocomposite light gas permeability increased to over four times that of the unfilled polymer, whereas at high periclase loadings the nanocomposites exhibited light gas permeabilities in excess of an order of magnitude higher than the unfilled materials. Even at these high loadings the light gas selectivities were higher than predicted for films containing transmembrane defects. Solubility was relatively unaffected by the void volume concentration, although it did increase to some extent depending on the nanoparticle concentration. Wide angle X-ray diffraction, nuclear magnetic resonance, and Fourier transform infra-red experiments were used to determine if the nanoparticles remained stable during film preparation. TiO₂ nanoparticles did not appear to react with water, the polymer matrixes or test gases used in this research. However, under certain circumstances, periclase reacted with adventitious water to form brucite. A desilylation reaction occurred when brucite was exposed to polymers or small molecule compounds that contained a trimethylsilyl group attached to a conjugated organic backbone. This reaction caused certain disubstituted polyacetylenes to become insoluble in common organic solvents.
Local electrostatic potential and strain characterization of semiconductor nanostructures
(2006) Chung, Jayhoon; Rabenberg, Llewellyn
Materials characterization techniques that determine the local charge transport properties of electronic devices are of increasing importance. Dopant profiles, electrostatic potential distributions near interfaces, strains, and defects within nanoscale devices should be characterized very locally, two-dimensionally and quantitatively for a complete understanding of device characteristics. Transmission electron microscopy (TEM) has the ability to resolve the atomic structure of materials, but conventional TEM methods do not give all the information that would be desirable for complete device characterization. This dissertation examines two advanced phase reconstruction techniques that can be used to characterize electrostatic potentials and strains in semiconductor nanostructures. Electron holography was utilized to measure the electrostatic potentials associated with charges and their distribution within a core/shell nanowire. Electron holography was optimized for the nanowire geometry using a dual-lens imaging configuration. Using this method, the mean inner potential of intrinsic germanium and its oxide were determined to be 15.50 ± 0.44 V and 9.10 ± 0.42 V, respectively. The B concentration within the B-doped shell of the core/shell nanowire was determined through a comparison of measured and simulated phase profiles. Fermi level pinning at interface states between the doped shell and the inner germanium oxide was also observed by electron holography. The screening length and the potential in the interface charge region were quantitatively measured. These characteristics compared favorable with the values obtained from numerical solution of Poisson’s equation. The local strain in a strained silicon (sSi) wafer was characterized using geometric phase analysis of high-resolution TEM (HRTEM) images. The method enables the reconstruction of strain maps from HRTEM images by digital image processing alone, when the HRTEM images were taken under careful controlled imaging conditions. Using specimens with known strain values, this method was confirmed to give a reliable, quantitative measure of strains in a sSi structure. Geometric phase analysis was also applied to real sSi layers grown on relaxed SiGe alloys containing either 43.9 or 17.7 atomic percent Ge. The defects and stress relaxation of these wafers were also analyzed.
Real-space pseudopotential calculations for the electronic and structural properties of nanostructures
(2010-08) Han, Jiaxin; Chelikowsky, James R.; Demkov, Alex; MacDonald, Allan; Korgel, Brian; Kleinman, Leonard
Nanostructures often possess unique properties, which may lead to the development of new microelectronic and optoelectronic devices. They also provide an opportunity to test fundamental quantum mechanical concepts such as the role of quantum confinement. Considerable effort has been made to understand the electronic and structural properties of nanostructures, but many fundamental issues remain. In this work, the electronic and structural properties of nanostructures are examined using several new computational methods. The effect of dimensional confinement on quantum levels is investigated for hydrogenated Ge <110> using the plane-wave density-functional-theory pseudopotential method. We present a real-space pseudopotential method for calculating the electronic structure of one-dimensional periodic systems such as nanowires. As an application of this method, we examine H-passivated Si nanowires. The band structure and heat of formation of the Si nanowires are presented and compared to plane wave methods. Our method is able to offer the same accuracy as the traditional plane wave methods, but offers a number of computational advantages such as the ability to handle large systems and a better ease of implementation for highly parallel platforms. Doping is important to many potential applications of nano-regime semiconductors. A series of first-principles studies are conducted on the P-doped Si <110> nanowires by the real-space pseudopotential methods. Nanowires of varied sizes and different doping positions are investigated. We calculate the binding energies of P atoms, band gaps of the wires, energetics of P atoms in different doping positions and core-level shift of P atoms. Defect wave functions of P atoms are also analyzed. In addition, we study the electronic properties of phosphorus-doped silicon <111> nanofilms using the real-space pseudopotential method. Nanofilms with varied sizes and different doping positions are investigated. We calculate the binding energies of P atoms, band gaps of the films, and energetics of P atoms in different doping positions. Quantum confinement effects are compared with P-doped Si nanocrystals and as well as nanowires. We simulate the nanofilm STM images with P defects in varied film depths, and make a comparison with the experimental measurement.
Schrödinger equation Monte Carlo simulation of nano-scaled semiconductor devices
(2004) Chen, Wanqiang; Register, Leonard F.
Semiconductor devices have been continuously scaled into the deep submicron regime. As a result, quantum effects which were neglected in semiclassical models become more and more important. Meanwhile, scattering still remains important down to the gate length around 10 nm. Accurate quantum transport simulators with scattering will be needed to explore the essential device physics. The work of this dissertation project is aimed at developing an accurate quantum transport simulation tool for deep submicron device modeling, as well as utilizing this newly developed simulation tool to study the quantum transport and scattering effects in ultra-scaled semiconductor devices. The quantum transport simulator “Schrödinger Equation Monte Carlo” (SEMC) provides a physically rigorous treatment of quantum transport and phasebreaking inelastic scattering (in 3D) via real (actual) scattering processes such as optical and acoustic phonon scattering. SEMC has been used to simulate carrier transport in nano-scaled devices in order to gauge the potential reliability of semiclassical models, phase-coherent quantum transport, and other limiting models as the transition from classical to quantum transport is approached. SEMC has also been successfully applied to study the carrier capture and transport in tunnel injection lasers. In this work, a 2D version of SEMC − SEMC-2D − has been developed. The quantum transport equations are solved self-consistently with Poisson equation. SEMC-2D has been used to simulate quantum transport in nano-scaled double gate MOSFETs. Simulation results serve not only to demonstrate the capability of this new quantum transport simulator, but also to illuminate the importance of physically accurate simulation of scattering for predictive modeling of transport in nano-scaled MOSFETs.
Synthesis and characterization of III-V semiconductor nanowires and fabrication of colloidal nanorod solar cells
(2006) Davidson, Forrest Murray; Korgel, Brian A.
Nanowires have attracted intensive research efforts due to their one-dimensional quantum confinement and their ability to serve as the building blocks for and functional components of future semiconductor devices. Widespread use of nanowires in bottom-up device fabrication will require a general method for the controllable synthesis of nanowires with regards to shape, size, composition, and interfacial properties. Gallium arsenide and gallium phosphide nanowires as small as 8 nm in diameter were synthesized in supercritical hexane and seeded by alkanethiol-stabilized 7 nm gold nanocrystals. The wires are single crystal with a zinc-blende structure and grow exclusively in the <111> direction. The importance of precursor degradation kinetics was explored. Multiple lamellar {111} twins are observed in GaAs, GaP and InAs nanowires synthesized by supercritical fluid-liquid-solid (SFLS) and solution-liquid-solid (SLS) approaches. All of these nanowires have zinc blende (cubic) crystal structure and were grown in the <111> direction. The twins cross-section the nanowires to give them a “bamboo”-like appearance in TEM images. In contrast, Si and Ge nanowires with <111> growth direction do not exhibit {111} twins, even though this is a common twin plane with relatively low twin energy in diamond cubic Ge and Si. However, Si and Ge nanowires with <112> growth directions typically have several {111} twins extending down the length of the nanowires. A semi-quantitative model that explains the observed twinning in III-V and IV nanowires is presented. Heterojunction solar cells were fabricated from colloidal solutions of CdTe and CdSe nanorods by sequential spin casting onto ITO coated glass substrates. A broad range of factors impacting the success of the solar cell fabrication were explored; including the method of nanorod synthesis, choice of capping ligand, method of active layer application, and use of hydrazine treatment. It is necessary to maximize the stability of the nanords in solution to ensure even application into thin films. However, electrical properties of the nanorod films are improved by using weaker stabilizing agents. The devices exhibit good diode behavior.
Synthesis and field emission studies of 1-D nanostructures
(2005) Kulkarni, Niraj Narasinha; Shih, Chih-Kang; Yao, Zhen, Ph. D.
1-D Nanostructures are attractive candidates for electron emitters in vacuum microelectronic devices because of their sharp tip radii and high aspect ratio. With advances in nanotechnology, various strategies have been reported for controlled synthesis of nanostructures including 1-D variants (nanowires and nanotubes). While various functional electronic/optoelectronic devices and circuits have been demonstrated using nanostructures, this work is focused on the synthesis and field emission studies of 1-D nanostructures of three materials systems, namely carbon nanotubes, silicon nanowires and graphitic nanocones. The potential applications of 1-D nanostructures as electron emitters are varied and include displays, microwave amplifiers, x-ray sources, holography, multiple e-beam lithography, electronic cooling. The carbon nanotubes (CNTs) are grown in anodic alumina templates by thermal chemical vapor deposition (CVD). The Silicon Nanowires (Si NWs) are grown by atmospheric pressure CVD (APCVD) via hydrogen reduction of silicon tetrachloride with an Au thin film acting as the catalyst for the Vapor-Liquid-Solid (VLS) process. Further post-growth processing was employed in the case of Si NWs, namely in-situ annealing and cesiation, to improve the field emission characteristics. Finally, field emission characterization of individual tubular graphitic nanocones (TGCs) was carried out. The TGCs were grown on iron needle by microwave plasma assisted CVD of C2H2 + N2. An individual nanocone emitted a current as high as 80 µA, corresponding to a current density of ~ 108 A/cm2 , without breakdown. Individual emitters would be of interest for applications in holography and as coherent electron sources.
Synthesis and thermoelectric properties of higher manganese silicides for waste heat recovery
(2014-12) Chen, Xi, active 21st century; Shi, Li, Ph. D.; Zhou, Jianshi
Thermoelectric (TE) materials, which can convert temperature gradients directly into electricity and vice versa, have received renewed interest for waste heat recovery and refrigeration applications. Higher manganese silicides (HMS) are promising p-type TE materials due to the abundance of the constituent elements, environmental friendliness, and good chemical stability. The objective of this dissertation is to establish a better understanding of the structure-TE properties relationship of HMS with a complex Nowotny chimney ladder structure. The focus of this work is on the investigations of the phonon dispersion of HMS crystals and the effects of chemical doping and nanostructuring on the TE properties of polycrystalline HMS. HMS crystals have been synthesized by the Bridgeman method for inelastic neutron scattering measurements of the phonon dispersion. In conjunction with density functional theory calculations, the results clearly show the presence of numerous low-lying optical phonon branches, especially an unusually low-energy optical phonon polarization associated with the twisting motions of the Si helical ladders in the Mn chimneys. The obtained phonon dispersion can be used to explain the low and anisotropic thermal conductivity of HMS crystals. (Al,Ge) double doping was found to be effective in modifying the electrical properties of HMS polycrystals. The peak thermoelectric power factor occurs at an optimized hole concentration of 1.8~2.2×10²¹ cm⁻³ at room temperature. On the other hand, Re substitution can suppress the lattice thermal conductivity to approach the calculated minimum value corresponding to the amorphous limit. Meanwhile, the thermoelectric power factor does not markedly change at low Re content of x ≤ 0.04 although it drops considerably with increasing Re content. Hence, the peak ZT has been improved to ~0.6 in both systems. The effects of nanostructuring on the TE properties have been studied in the cold-pressed samples and ball-milled samples. The thermal conductivity was reduced remarkably by decreasing the grain size. It is found that the grain size effects are more significant at low temperature. However, it is difficult to reduce the grain size to less than 50 nm without the formation of impurity phases by ball milling. These facts limit the ZT enhancement of the nanostructured HMS at high temperatures in this study.
Synthesis of silicon/germanium nanowires and field emission studies of 1-D nanostructures
(2007-05) Bae, Joonho, 1972-; Shih, Chih-Kang
Using the vapor-liquid-solid (VLS) growth method, silicon nanowires and germanium nanowires are grown. We find the high growth rate is responsible for the silicon nanowires with less growth defects when they are grown by use of silicon tetrachloride as a precursor and hydrogen as a carrier gas. Based on this funding, large area, high aspect ratio, h111i oriented silicon nanowires are successfully grown on Si (111) and Si (100). Novel growth mechanisms of VLS growth method were discovered in SiOx nanoflowers and silicon nanocones. In SiOx nanoflowers grown at the tip of silicon nanowires, it is found that they are produced via the enhanced oxidation of silicon at the gold-silicon interface. Furthermore, the analysis of the flower pattern reveals that it is the observation of the dense branching morphology on nanoscale and on spherical geometry. For the silicon nanocones, they are grown by the in situ etching of the catalysts of Ga/Al by HCl during the growth. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) reveal that the nanocones are composed of amorphous silicon oxides and crystalline Si. Based on the similar chemistry of hydrogen reduction of SiCl₄ for the growth of silicon nanowires, single crystalline germanium nanowires are grown by use of GeCl4 as a precursor and H₂ as a carrier gas. As one of important application of one dimensional nanostructures, the field emission properties of 1-D nanostructures are explored. The field emission properties of a single graphite nanocone are measured in SEM. The inter-electrode separation is controlled using scanning tunneling microscopy (STM) approach method, allowing the precise and ne determination of the separation. Its Fowler-Nordheim plot shows it emits currents in accordance with the Fowler-Nordheim field emission. Its onset voltage, field enhancement factor show that its basic field emission parameters are comparable to those of a single carbon nanotube. It is observed that single nanocone is damaged after emitting a current of about 100 nA, which seems to be due to its hollow interior structure.
The development of scalable, self-assembled, low-cost nanophotonic structures for enhanced photocurrent generation for light-weight, portable, and high-efficiency III-V solar cells
(2021-01-21) Cossio, Gabriel; Yu, Edward T.; Korgel, Brian A; Bonnecaze, Roger T; Sreenivasan, S V; Bank, Seth R
Nanosphere lithography is a versatile and low-cost method for defining two dimensional periodic lithographic patterns on the nanoscale. However, nanosphere lithography suffers from a lack of scalability that has prevented its low-cost nature from being successfully integrated into any commercialized processes. This dissertation investigates methods to improve the scalability of nanosphere lithography in order to enable the development of low-cost nanophotonic light management structures which increase the photogenerated current in portable, light-weight, high-efficiency III-V solar cells. A new physical model for monolayer self-assembly via the injection of colloidal nanoparticles onto an air-water interface is presented. The physical model is verified by experiment and shows that particle-particle interactions play an important role in increasing the growth rate of self-assembled nanoparticle monolayers on an air-water interface. Experiments show that by increasing particle closeness, as measured by the magnitude of the zeta potential, a ~7x improvement in monolayer growth-rate can be achieved. The physical model is verified for the injected self-assembly of polystyrene and SiO2 nanospheres monolayers, as well as for particles in the range of 200 nm – 2μm. Experimental techniques are presented for the successful transfer of large area (>200cm2) self-assembled nanosphere monolayers onto rigid or flexible substrates, demonstrating that the scalability of nanosphere lithography is in fact possible. The improved nanosphere lithography method is utilized to fabricate large-area moth-eye nanopatterned on PET polymer packaging sheets for flexible solar modules. The moth-eye textured PET sheets are optically characterized and are shown to provide broadband, omnidirectional reduction in specular and diffuse reflectance. The moth-eye textured PET sheets are shown to survive an industrial lamination procedure and are utilized to encapsulate a light-weight, portable, flexible solar module made from an array of triple junction III-V based epitaxial lift-off solar cells. Maximum photogenerated current enhancement due to the omnidirectional antireflection properties of the moth-eye textured encapsulant is measured to be ~60% higher at ~80o angle of incidence relative to a solar module without any nanotexturing of the PET packaging layer. Lastly, future work that can benefit from the enhanced nanosphere lithography method are outlined, particularly the fabrication of low-cost self-assembled light trapping structures.
Transition metal dichalcogenide MoSe2 nanostructures
(2016-12-13) Chen, Yuxuan, 1986-; Shih, Chih-Kang; de Lozanne, Alejandro; Fiete, Gregory A; Niu, Qian; Shi, Li
Transition metal dichalcogenides (TMDs) are a family of van der Waals (vdW) layered materials exhibiting unique electronic, optical, magnetic, and transport properties. Their technological potentials hinge critically on the ability to achieve controlled fabrication of desirable nanostructures. Here I present three kinds of nanostructures of semiconducting TMD MoSe₂, created by molecular beam epitaxy (MBE) and characterized by scanning tunneling microscopy and spectroscopy (STM/STS). The three kinds of nanostructures are two-dimensional (2D) nanoislands, quasi one-dimensional (1D) nanoribbons, and heterostructures. The successful growth of 2D nanoislands lays the foundation for the preparation of the other two structures. By properly controlling the substrate temperature and Se over-pressure, the MoSe₂ atomic layers undergo a dramatic three-stage shape transformation: from fractal to compact 2D nanoislands, and eventually to nanoribbons, in stark contrast to the traditional two-stage growth behaviour involving only the transformation from the fractal to compact regime. Experimentally, it is found that the Se:Mo flux ratio during MBE growth plays a central role in controlling the nanoribbon formation. Theoretically, first-principles calculations show that the abundance/deficiency of extra Se atoms at different island edges significantly modifies the relative step energies between zigzag and armchair edges, which in turn impacts the island shape evolution during nonequilibrium growth. The successful preparation of MoSe2/hBN/Ru(0001) heterostructure is a demonstration that MBE technique is suitable for fabricating vdW heterostructures. Surprisingly, we found that the quasi-particle gap of the MoSe₂ on hBN/Ru is about 0.25 eV smaller than those on graphene or graphite substrates. We attribute this result to the strong interaction between hBN/Ru which causes residual metallic screening from the substrate. The surface of MoSe₂ exhibits Moiré pattern that replicates the Moiré pattern of hBN/Ru. In addition, the electronic structure and the work function of MoSe₂ are modulated electrostatically with an amplitude of ~ 0.13 eV. Most interestingly, this electrostatic modulation is spatially in phase with the Moiré pattern of hBN on Ru(0001) whose surface also exhibits a work function modulation of the same amplitude