Wooyoung Shim

Hard-tip, soft-spring lithography

Nanofabrication strategies are becoming increasingly expensive and equipment-intensive, and consequently less accessible to researchers. As an alternative, scanning probe lithography has become a popular means of preparing nanoscale structures, in part owing to its relatively low cost and high resolution, and a registration accuracy that exceeds most existing technologies. However, increasing the throughput of cantilever-based scanning probe systems while maintaining their resolution and registration advantages has from the outset been a significant challenge. Even with impressive recent advances in cantilever array design, such arrays tend to be highly specialized for a given application, expensive, and often difficult to implement. It is therefore difficult to imagine commercially viable production methods based on scanning probe systems that rely on conventional cantilevers. Here we describe a low-cost and scalable cantilever-free tip-based nanopatterning method that uses an array of hard silicon tips mounted onto an elastomeric backing. This method—which we term hard-tip, soft-spring lithography—overcomes the throughput problems of cantilever-based scanning probe systems and the resolution limits imposed by the use of elastomeric stamps and tips: it is capable of delivering materials or energy to a surface to create arbitrary patterns of features with sub-50-nm resolution over centimetre-scale areas. We argue that hard-tip, soft-spring lithography is a versatile nanolithography strategy that should be widely adopted by academic and industrial researchers for rapid prototyping applications.

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.

Scanning probe block copolymer lithography

Integration of individual nanoparticles into desired spatial arrangements over large areas is a prerequisite for exploiting their unique electrical, optical, and chemical properties. However, positioning single sub-10-nm nanoparticles in a specific location individually on a substrate remains challenging. Herein we have developed a unique approach, termed scanning probe block copolymer lithography, which enables one to control the growth and position of individual nanoparticles in situ. This technique relies on either dip-pen nanolithography (DPN) or polymer pen lithography (PPL) to transfer phase-separating block copolymer inks in the form of 100 or more nanometer features on an underlying substrate. Reduction of the metal ions via plasma results in the high-yield formation of single crystal nanoparticles per block copolymer feature. Because the size of each feature controls the number of metal atoms within it, the DPN or PPL step can be used to control precisely the size of each nanocrystal down to 4.8 ± 0.2 nm.

Matrix‐Assisted Dip‐Pen Nanolithography and Polymer Pen Lithography

The controlled patterning of nanomaterials presents a major challenge to the field of nanolithography because of differences in size, shape and solubility of these materials. Matrix-assisted dip-pen nanolithography and polymer pen lithography provide a solution to this problem by utilizing a polymeric matrix that encapsulates the nanomaterials and delivers them to surfaces with precise control of feature size.

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 ]

Desktop Nanofabrication with Massively Multiplexed Beam Pen Lithography

The development of a lithographic method that can rapidly define nanoscale features across centimeter-scale surfaces has been a long standing goal of the nanotechnology community. If such a ‘desktop nanofab’ could be implemented in a low-cost format, it would bring the possibility of point-of-use nanofabrication for rapidly prototyping diverse functional structures. Here we report the development of a new tool that is capable of writing arbitrary patterns composed of diffraction-unlimited features over square centimeter areas that are in registry with existing patterns and nanostructures. Importantly, this instrument is based on components that are inexpensive compared to the combination of state-of-the-art nanofabrication tools that approach its capabilities. This tool can be used to prototype functional electronic devices in a mask-free fashion in addition to providing a unique platform for performing high throughput nano- to macroscale photochemistry with relevance to biology and medicine.

Arrays of Nanoscale Lenses for Subwavelength Optical Lithography

Poly(ethylene glycol) (PEG) polymer lens arrays are made by using dip-pen nanolithography to deposit nanoscale PEG features on hydrophobically modified quartz glass. The dimensions of the PEG lenses are controlled by tuning dwell time and polymer molecular weight. The PEG polymer lenses on the quartz substrate act as a phase-shift photomask for fabricating subwavelength scale features, ∼ 100 nm in width. Depending upon UV irradiation time during the photolithography, the photoresist nanostructures can be transitioned from well-shaped (short time) to ring-shaped (long time) features. The technique can be used to pattern large areas through the use of cantilever arrays.

Design Rules for Nanogap‐Based Hydrogen Gas Sensors

Nanoscale gaps, which enable many research applications in fields such as chemical sensors, single-electron transistors, and molecular switching devices, have been extensively investigated over the past decade and have witnessed the evolution of related technologies. Importantly, nanoscale gaps employed in hydrogen-gas (H(2)) sensors have been used to reversibly detect H(2) in an On-Off manner, and function as platforms for enhancing sensing performance. Herein, we review recent advances in nanogap design for H(2) sensors and deal with various strategies to create these gaps, including fracture generation by H(2) exposure, deposition onto prestructured patterns, island formation on a surface, artificial manipulation methods, methods using hybrid materials, and recent approaches using elastomeric substrates. Furthermore, this review discusses a new nanogap design that advances sensing capabilities in order to meet the diverse needs of academia and industry.

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.