A. Alec Talin

Stress-induced chemical detection using flexible metal-organic frameworks.

In this work we demonstrate the concept of stress-induced chemical detection using metal-organic frameworks (MOFs) by integrating a thin film of the MOF HKUST-1 with a microcantilever surface. The results show that the energy of molecular adsorption, which causes slight distortions in the MOF crystal structure, can be converted to mechanical energy to create a highly responsive, reversible, and selective sensor. This sensor responds to water, methanol, and ethanol vapors, but yields no response to either N2 or O2. The magnitude of the signal, which is measured by a built-in piezoresistor, is correlated with the concentration and can be fitted to a Langmuir isotherm. Furthermore, we show that the hydration state of the MOF layer can be used to impart selectivity to CO2. Finally, we report the first use of surface-enhanced Raman spectroscopy to characterize the structure of a MOF film. We conclude that the synthetic versatility of these nanoporous materials holds great promise for creating recognition chemistries to enable selective detection of a wide range of analytes.

A roadmap to implementing metal-organic frameworks in electronic devices: challenges and critical directions.

Metal-organic frameworks (MOFs) and related material classes are attracting considerable attention for applications such as gas storage, separations, and catalysis. In contrast, research focused on potential uses in electronic devices is in its infancy. Several sensing concepts in which the tailorable chemistry of MOFs is used to enhance sensitivity or provide chemical specificity have been demonstrated, but in only a few cases are MOFs an integral part of an actual device. The synthesis of a few electrically conducting MOFs and their known structural flexibility suggest that MOF-based electronic devices exploiting these properties could be constructed. It is clear, however, that new fabrication methods are required to take advantage of the unique properties of MOFs and extend their use to the realms of electronic circuitry. In this Concepts article, we describe the basic functional elements needed to fabricate electronic devices and summarize the current state of relevant MOF research, and then review recent work in which MOFs serve as active components in electronic devices. Finally, we propose a high-level roadmap for device-related MOF research, the objective of which is to stimulate thinking within the MOF community concerning the development these materials for applications including sensing, photonics, and microelectronics.

Silver Cluster Formation, Dynamics, and Chemistry in Metal−Organic Frameworks

Synthetic methods used to produce metal nanoparticles typically lead to a distribution of particle sizes. In addition, creation of the smallest clusters, with sizes of a few to tens of atoms, remains very challenging. Nanoporous metal-organic frameworks (MOFs) are a promising solution to these problems, since their long-range crystalline order creates completely uniform pore sizes with the potential for both steric and chemical stabilization. We report a systematic investigation of silver nanocluster formation within MOFs using three representative MOF templates. The as-synthesized clusters are spectroscopically consistent with dimensions < or =1 nm, with a significant fraction existing as Ag(3) clusters, as shown by electron paramagnetic resonance. Importantly, we show conclusively that very rapid TEM-induced MOF degradation leads to agglomeration and stable, easily imaged particles, explaining prior reports of particles larger than MOF pores. These results solve an important riddle concerning MOF-based templates and suggest that heterostructures composed of highly uniform arrays of nanoparticles within MOFs are feasible.

The relationship between the spatially resolved field emission characteristics and the raman spectra of a nanocrystalline diamond cold cathode

Spatially resolved electron field emission measurements from a nanocrystalline diamond film grown by plasma‐enhanced chemical transport deposition have been obtained using a scanning probe apparatus with micrometer resolution. Macroscopic regions with a high emission site density, and turn‐on fields below 3 V/μm, comprised approximately 1/2 of the total sample area. The emitting and the nonemitting regions of the specimen are differentiated distinctly by Raman spectra and subtly by morphologies. Both areas are largely sp3‐bonded, but only the nonemitting regions exhibit a sharp line at 1332 cm−1, a well‐known signature of diamond in larger crystallites.

Highly aligned, template-free growth and characterization of vertical GaN nanowires on sapphire by metal–organic chemical vapour deposition

We report the growth of exceptionally well aligned and vertically oriented GaN nanowires on r-plane sapphire wafers via metal–organic chemical vapour deposition. The nanowires were grown without the use of either a template or patterning. Transmission electron microscopy indicates the nanowires are single crystalline, free of threading dislocations, and have triangular cross-sections. The high degree of vertical alignment is explained by the crystallographic match between the oriented nanowires and the r-plane sapphire surface. We find that the degree of alignment and size uniformity of the nanowires are highly dependent on the nickel nitrate catalyst concentration used, with the highest degree of uniformity and alignment occurring at concentrations much more dilute than typically employed for vapour–liquid–solid-based nanowire growth. Additionally, we report here a strong dependence of the optical and electrical properties of the nanowires on the growth temperature, which we hypothesize is due to increased carbon incorporation at lower growth temperatures.

Size-dependent effects on electrical contacts to nanotubes and nanowires

Metal-semiconductor contacts play a key role in electronics. Here we show that for quasi-one-dimensional (Q1D) structures such as nanotubes and nanowires, side contact with the metal only leads to weak band realignment, in contrast with bulk metal-semiconductor contacts. Schottky barriers are much reduced compared with the bulk limit, and should facilitate the formation of good contacts. However, the conventional strategy of heavily doping the semiconductor to obtain Ohmic contacts breaks down as the nanowire diameter is reduced. The issue of Fermi level pinning is also discussed, and it is demonstrated that the unique density of states of Q1D structures makes them less sensitive to this effect. Our results agree with recent experimental work, and should apply to a broad range of Q1D materials.

Scanning tunneling microscopy of DNA-wrapped carbon nanotubes.

We employ scanning tunneling microscopy (STM) to reveal the structure of DNA-carbon nanotube complexes with unprecedented spatial resolution and compare our experimental results to molecular dynamics simulations. STM images show strands of DNA wrapping around (6,5) nanotubes at approximately 63 degrees angle with a coiling period of 3.3 nm, in agreement with the theoretical predictions. In addition, we observe width modulations along the DNA molecule itself with characteristic lengths of 1.9 and 2.5 nm, which remain unexplained. In our modeling we use a helical coordinate system, which naturally accounts for tube chirality along with an orbital charge density distribution and allows us to simulate this hybrid system with the optimal pi-interaction between DNA bases and the nanotube. Our results provide novel insight into the self-assembling mechanisms of nanotube-DNA hybrids and can be used to guide the development of novel DNA-based nanotube separation and self-assembly methods, as well as drug delivery and cancer therapy techniques.

A Perspective on Nanowire Photodetectors: Current Status, Future Challenges, and Opportunities

One-dimensional semiconductor nanostructures (nanowires (NWs), nanotubes, nanopillars, nanorods, etc.) based photodetectors (PDs) have been gaining traction in the research community due to their ease of synthesis and unique optical, mechanical, electrical, and thermal properties. Specifically, the physics and technology of NW PDs offer numerous insights and opportunities for nanoscale optoelectronics, photovoltaics, plasmonics, and emerging negative index metamaterials devices. The successful integration of these NW PDs on CMOS-compatible substrates and various low-cost substrates via direct growth and transfer-printing techniques would further enhance and facilitate the adaptation of this technology module in the semiconductor foundries. In this paper, we review the unique advantages of NW-based PDs, current device integration schemes and practical strategies, recent device demonstrations in lateral and vertical process integration with methods to incorporate NWs in PDs via direct growth (nanoepitaxy) methods and transfer-printing methods, and discuss the numerous technical design challenges. In particular, we present an ultrafast surface-illuminated PD with 11.4-ps full-width at half-maximum (FWHM), edge-illuminated novel waveguide PDs, and some novel concepts of light trapping to provide a full-length discussion on the topics of: 1) low-resistance contact and interfaces for NW integration; 2) high-speed design and impedance matching; and 3) CMOS-compatible mass-manufacturable device fabrication. Finally, we offer a brief outlook into the future opportunities of NW PDs for consumer and military application.

Unusually strong space-charge-limited current in thin wires.

The current-voltage characteristics of thin wires are often observed to be nonlinear, and this behavior has been ascribed to Schottky barriers at the contacts. We present electronic transport measurements on GaN nanorods and demonstrate that the nonlinear behavior originates instead from space-charge-limited current. A theory of space-charge-limited current in thin wires corroborates the experiments and shows that poor screening in high-aspect ratio materials leads to a dramatic enhancement of space-charge limited current, resulting in new scaling in terms of the aspect ratio.

Large area, dense silicon nanowire array chemical sensors

The authors present a simple top-down approach based on nanoimprint lithography to create dense arrays of silicon nanowires over large areas. Metallic contacts to the nanowires and a bottom gate allow the operation of the array as a field-effect transistor with very large on/off ratios. When exposed to ammonia gas or cyclohexane solutions containing nitrobenzene or phenol, the threshold voltage of the field-effect transistor is shifted, a signature of charge transfer between the analytes and the nanowires. The threshold voltage shift is proportional to the Hammett parameter and the concentration of the nitrobenzene and phenol analytes.