Jinhee Ham

On-Film Formation of Bi Nanowires with Extraordinary Electron Mobility

A novel stress-induced method to grow semimetallic Bi nanowires along with an analysis of their transport properties is presented. Single crystalline Bi nanowires were found to grow on as-sputtered films after thermal annealing at 260-270 degrees C. This was facilitated by relaxation of stress between the film and the thermally oxidized Si substrate that originated from a mismatch of the thermal expansion. The diameter-tunable Bi nanowires can be produced by controlling the mean grain size of the film, which is dependent upon the thickness of the film. Four-terminal devices based on individual Bi nanowires were found to exhibit very large transverse and longitudinal ordinary magnetoresistance, indicating high-quality, single crystalline Bi nanowires. Unusual transport properties, including a mobility value of 76900 cm(2)/(V s) and a mean free path of 1.35 mum in a 120 nm Bi nanowire, were observed at room temperature.

Direct Growth of Compound Semiconductor Nanowires by On-Film Formation of Nanowires: Bismuth Telluride

Bismuth telluride (Bi(2)Te(3)) nanowires are of great interest as nanoscale building blocks for high-efficiency thermoelectric devices. Their low-dimensional character leads to an enhanced figure-of-merit (ZT), an indicator of thermoelectric efficiency. Herein, we report the invention of a direct growth method termed On-Film Formation of Nanowires (OFF-ON) for making high-quality single-crystal compound semiconductor nanowires, that is, Bi(2)Te(3), without the use of conventional templates, catalysts, or starting materials. We have used the OFF-ON technique to grow single crystal compound semiconductor Bi(2)Te(3) nanowires from sputtered BiTe films after thermal annealing at 350 degrees C. The mechanism for wire growth is stress-induced mass flow along grain boundaries in the polycrystalline film. OFF-ON is a simple but powerful method for growing perfect single-crystal compound semiconductor nanowires of high aspect ratio with high crystallinity that distinguishes it from other competitive growth approaches that have been developed to date.

Reduction of Lattice Thermal Conductivity in Single Bi‐Te Core/Shell Nanowires with Rough Interface

Reducing the thermal conductivity of nanometer-scale materials is of signifi cant interest for a broad range of applications in the dissipation of heat from electronics and optoelectronics, and in thermoelectric energy conversion. When the relevant length scale of a nanostructure is comparable to the mean free path of the heat carriers, the heat transport can be effectively controlled, which often results in the reduction of the thermal conductivity of the nanostructure compared to its bulk counterpart. The reduced thermal conductivity provides an effective strategy for optimizing thermoelectric energy conversion as well as for managing heat generated in electronic and photonic devices. Considerable effort has been invested in developing methods to reduce thermal conductivity, largely because the reduction of thermal conductivity helps increase the thermoelectric fi gureof-merit ( ZT ) (defi ned as ZT = S 2 σ T/ κ , where S , σ , κ , and T are the Seebeck coeffi cient, electrical conductivity, thermal conductivity, and absolute temperature, respectively). [ 1 ] In this respect, rational synthetic routes for κ reduction include the insertion of nanometer scale inclusions in bulk materials, [ 2 ] the epitaxial growth of superlattice thin fi lms, [ 3 ] one-dimensional heterostructures, [ 4,5 ] and the use of photonic nanomesh structures. [ 6,7 ] A particularly versatile technique is to introduce a rough surface on silicon nanowires, [ 8 ] which provides effi cient scattering across the broad phonon spectrum, and thus reduces κ as much as two orders of magnitude relative to bulk crystalline silicon. In a core/shell structure, which is low-dimensional heterostructures, has the signifi cant advantage in the enhancement of ZT owing to low thermal conductivity by interface phonon scattering. [ 4,5 ]

Direct observation of the semimetal-to-semiconductor transition of individual single-crystal bismuth nanowires grown by on-film formation of nanowires

We have systematically investigated the semimetal-to-semiconductor transition of individual single-crystalline Bi nanowires. For this work, we developed a technique to reduce the diameter of Bi nanowires grown by our unique on-film formation of nanowires (OFF-ON) method. Cooling down the substrate temperature during Bi film deposition by use of liquid nitrogen, film structures with small-sized grains were obtained. Through thermal annealing of these fine-granular Bi films, single-crystalline Bi nanowires can be produced with minimum diameter of approximately 20 nm. Elaborative nanofabrication techniques were employed to shape state-of-the-art four-probe devices based on the individual small diameter Bi nanowires. Diameter-dependent transport measurements on the individual Bi nanowires revealed that the semimetal-to-semiconductor transition really occurred at about d(w) = 63 nm. Moreover, band structure calculations supported this occurrence of the semimetal-to-semiconductor transition.

Shubnikov–de Haas oscillations in an individual single-crystalline bismuth nanowire grown by on-film formation of nanowires

Shubnikov–de Haas (SdH) oscillations have been investigated in an individual Bi nanowire grown by on-film formation of nanowires that is a growth method producing extremely high-quality single-crystalline nanowires. The variation of observed SdH oscillations with transverse and longitudinal magnetic fields to the axis of the Bi nanowire is qualitatively consistent with the geometry of the highly anisotropic Fermi surfaces of Bi, and in turn, reveals the growth direction of the nanowires and demonstrates the high crystal quality. Our results demonstrate the vast potential of high-quality single-crystalline Bi nanowires for a variety of device applications and for fundamental investigations such as quantum transport.

Watching bismuth nanowires grow

We report real-time high temperature scanning electron microscopy observations of the growth of bismuth nanowires via the on-film formation of nanowires (OFF-ON) method. These observations provide experimental evidence that thermally induced-stress on a Bi film is the driving force for the growth of Bi nanowires with high aspect ratios, uniform diameter, and high-quality crystallinity. Our results show that immobile grain boundaries in the Bi film are required for the growth of nanowires so that grain broadening resulting in hillock formation can be prevented. This study not only provides an understanding of the underlying mechanism, but also affords a strategy for facilitating nanowire growth by OFF-ON.

Magnetotransport properties of an individual single-crystalline Bi nanowire grown by a stress induced method

The magnetotransport properties of an individual crystalline Bi nanowire have been investigated in the range of 2–300 K using four-point measurements. I-V measurements show that the contacts were Ohmic at both 2 and 300 K, corresponding to resistivities of 1.0×10−4 and 8.2×10−5 Ω cm, respectively. The transverse magnetoresistance (MR) (2496% at 110 K) and longitudinal MR (−38% at 2 K) for the Bi nanowire were found to be larger than any values reported in the literature, demonstrating that the Bi nanowires grown by a stress induced method are high-quality single crystalline. The observed transverse and longitudinal MR behaviors in the Bi nanowire are consistent with variations in carrier concentrations as well as electronic structures, such as Fermi level and band overlap, based on simple two band model.

Self-assembled Bi interconnections produced by on-film formation of nanowires for in situ device fabrication

We fabricated Bi nanowire interconnections between two pre-patterned electrodes using a combination of on-film formation of nanowires (OFF-ON) and self-assembly. Bi nanowires were found to grow laterally from a multilayer structure with a Cr (or SiO(2)) overlayer on top of a Bi thin film through thermal annealing to relieve vertically stored compressive stress. A Bi nanobridge with a diameter of 192 nm was formed between two Cr electrodes and was highly ohmic according to I-V measurements. A high transverse magnetoresistance of 123% was also observed at 300 K. Our results indicate that self-assembled lateral nanowire growth can be utilized as an easy means for fabricating a variety of nanowire devices without the use of catalysts or complex patterning processes.

Structure-dependent growth control in nanowire synthesis via on-film formation of nanowires

On-film formation of nanowires, termed OFF-ON, is a novel synthetic approach that produces high-quality, single-crystalline nanowires of interest. This versatile method utilizes stress-induced atomic mass flow along grain boundaries in the polycrystalline film to form nanowires. Consequently, controlling the magnitude of the stress induced in the films and the microstructure of the films is important in OFF-ON. In this study, we investigated various experimental growth parameters such as deposition rate, deposition area, and substrate structure which modulate the microstructure and the magnitude of stress in the films, and thus significantly affect the nanowire density. We found that Bi nanowire growth is favored in thermodynamically unstable films that facilitate atomic mass flow during annealing. A large film area and a large thermal expansion coefficient mismatch between the film and the substrate were found to be critical for inducing large compressive stress in a film, which promotes Bi nanowire growth. The OFF-ON method can be routinely used to grow nanowires from a variety of materials by tuning the material-dependent growth parameters.

Co nanoparticle hybridization with single-crystalline Bi nanowires

Crystalline Co nanoparticles were hybridized with single-crystalline Bi nanowires simply by annealing Co-coated Bi nanowires at elevated temperatures. An initially near-amorphous Co film of 2-7 nm in thickness began to disrupt its morphology and to be locally transformed into crystallites in the early stage of annealing. The Co film became discontinuous after prolonged annealing, finally leading to isolated, crystalline Co nanoparticles of 8-27 nm in size. This process spontaneously proceeds to reduce the high surface tension and total energy of Co film. The annealing time required for Co nanoparticle formation decreased as annealing temperature increased, reflecting that this transformation occurs by the diffusional flow of Co atoms. The Co nanoparticle formation process was explained by a hole agglomeration and growth mechanism, which is similar to the model suggested by Brandon and Bradshaw, followed by the nanoparticle refinement.