Marcus D. Lay

Preparation and modification of carbon nanotubes: review of recent advances and applications in catalysis and sensing.

Single-walled carbon nanotubes (SWNTs) have become one of the most intensely studied nanostructures because of their unique properties. The inherent physical properties of carbon nanotubes make them ideal supports for metal nanoparticles. The use of electrodeposition to modify SWNTs in order to facilitate applications in areas related to catalysis and sensing is presented in this manuscript. Preparation of raw SWNT material for electrochemical experiments involves various mild or oxidative pretreatments. In this review we focus on progress toward functionalization of SWNTs with metal nanoparticles using electrochemical methods and the applications of metal decorated carbon nanotubes in energy related applications.

1∕f noise in single-walled carbon nanotube devices

We report the scaling behavior of 1∕f noise in single-walled carbon nanotube devices. In this study we use two-dimensional carbon nanotube networks to explore the geometric scaling of 1∕f noise and find that for devices of a given resistance the noise scales inversely with device size. We have established an empirical formula that describes this behavior over a wide range of device parameters that can be used to assess the noise characteristics of carbon nanotube-based electronic devices and sensors.

Carbon nanotube networks: Nanomaterial for macroelectronic applications

We describe the properties and potential applications of an electronic material that consists of an interconnected random network of single-walled carbon nanotubes. This material possesses useful electronic properties, and it can be patterned into devices with high yield using conventional microfabrication technology. One unique aspect of this material is that every atom is a surface atom. For this reason nanotube networks form an ideal electronic material to utilize interface phenomena to engineer its properties for specific applications. We discuss two such applications: chemical sensing and macroelectronics.

Atomic layer epitaxy of CdTe using an automated electrochemical thin-layer flow deposition reactor☆

A number of different cycle chemistries, along with an automated thin-layer flow cell electrodeposition system, are described for the formation of CdTe thin film deposits using electrochemical atomic layer epitaxy (EC-ALE). Atomic layer epitaxy (ALE) involves the deposition of a compound one atomic layer at a time, via surface limited reactions, in a repeating cycle. In EC-ALE, underpotential deposition (upd) is used to form the atomic layers. Previous studies of the EC-ALE growth of CdTe have involved a cycle where Cd was deposited by reductive upd, followed by oxidative upd of Te from an acidic (pH 2.0) solution. In the present study, basic (pH 10.2) tellurium solutions were investigated in an attempt to use direct reductive upd of Te, as well as reductive upd of Cd. The idea was to simplify the cycle. The deposition in the basic solution is shifted dramatically negative, such that surface limited reductive deposition of Te appears to coincide with potentials used for reductive Cd upd, thus allowing both elements to be reductively deposited in a cycle at similar potentials. Improvements have been made relative to previous deposits reported by this group, such as an increase in the amount deposited per cycle. The old cycle and the H-cell design produced only 0.4 ML per cycle, while our new cycle deposits the expected 1 ML per cycle. However, there were some drawbacks to the new cycle, which was based on the reductive upd formation for both Cd and Te. Even though voltammetry for Te deposition on Au suggests that Te deposits by a surface limited process, it in fact deposits at an overpotential. Therefore, some bulk Te is inevitably deposited along with each Te atomic layer. The amount of bulk deposited is a function of convection in the cell, and thus leads to inhomogeneity in the deposit, something not expected for a purely surface limited process. In order to avoid the traces of bulk Te, the best deposits were formed when the reductive deposition of Te was combined with a bulk Te stripping step to remove excess material. This process is referred to here as oxidative Te upd. The resulting deposits evidenced a predominant [111] orientation for zinc blende CdTe (from XRD), and a band gap of 1.55 eV (from reflection adsorption measurements), consistent with the literature bandgap for CdTe.

Formation of the first monolayers of CdTe on Au(111) by electrochemical atomic layer epitaxy (EC-ALE): studied by LEED, Auger, XPS, and in-situ STM

Abstract The two-step alternated electrodeposition of Cd and Te atomic layers to form CdTe monolayers, electrochemical atomic layer epitaxy (EC-ALE), was studied on Au(111) using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), in-situ scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS). Well ordered (√7×√7) R 19.1°-CdTe and (3×3)-CdTe structures were formed using either Te or Cd as the first layer, and model structures are proposed for both. STM images suggest that previously proposed hexagonal structures based on a plane of zinc blende CdTe may be incorrect. A chain structure is suggested to account for the (√7×√7) R 19.1°, based on 3/7 (0.43) ML each of Cd and Te. The (3×3)-CdTe structure results from the deposition of a CdTe sandwich: first an atomic layer of Cd on Au, followed by a layer of Te, and then a second Cd layer. Based on this three layer model for the (3×3), a three step deposit was formed, starting with Cd, then Te, and finally Cd. This resulted in an excellent quality LEED pattern, suggesting a well-ordered (3×3)-CdTe deposit and strongly supporting the proposed model. The importance of deposit stoichiometry was also investigated using STM, which indicated that too low a coverage in the first atomic layer resulted in cluster formation and the degradation of surface morphology.

EC-STM Studies of Te and CdTe Atomic Layer Formation from a Basic Te Solution

The cyclic voltammetry of Te on Au is markedly affected by pH. This fact can be used to advantage when designing an electrodeposition cycle for CdTe. For instance, if a pH 2 Te solution is used, the underpotential deposition (UPD) potential for Te is 0.8 V positive of that for Cd. However, if a pH 9.2 Te deposition solution is used, the potential for Te UPD coincides with that for Cd, greatly simplifying the development of an electrochemical atomic layer epitaxy (EC-ALE) cycle. This report describes electrochemical scanning tunneling microscopy (EC-STM) studies of Te deposition on Au(111) and Au(100) from basic media. Several structures were observed on Au(111): a (6 × 6 ) tellurite adlayer which spontaneously adsorbed prior to Te formation, a 1/4 coverage (2 × 2)-Te, and a 1/3 coverage (2 X 10)-Te. While on Au(111), a 1/3 coverage (√3 × √3)-Te with (13 × 13) light domain walls, and two (3 × 3)-Te structures, with coverages of 4/9 and 5/9, were observed. Results of the formation of the first CdTe compound monolayer using an EC-ALE cycle which includes Te deposition from a basic solution are also included.

Bulk Purification and Deposition Methods for Selective Enrichment in High Aspect Ratio Single-Walled Carbon Nanotubes

Aqueous batch processing methods for the concurrent purification of single-walled carbon nanotube (SWNT) soot and enrichment in high aspect ratio nanotubes are essential to their use in a wide variety of electronic, structural, and mechanical applications. This manuscript presents a new route to the bulk purification and enrichment of unbundled SWNTs having average lengths in excess of 2 μm. Iterative centrifugation cycles at low centripetal force not only removed amorphous C and catalyst nanoparticles but also allowed the enhanced buoyancy of surfactant encapsulated, unbundled, high aspect ratio SWNTs to be used to isolate them in the supernatant. UV-vis-NIR and Raman spectroscopy were used to verify the removal of residual impurities from as-produced (AP-grade) arc discharge soot and the simultaneous enrichment in unbundled, undamaged, high aspect ratio SWNTs. The laminar flow deposition process (LFD) used to form two-dimensional networks of SWNTs prevented bundle formation during network growth. Additionally, it further enhanced the quality of deposits by taking advantage of the inverse relationship between the translational diffusion coefficient and length for suspended nanoparticles. This resulted in preferential deposition of pristine, unbundled, high aspect ratio SWNTs over residual impurities, as observed by Raman spectroscopy and atomic force microscopy (AFM).

Reducing electrical resistance in single-walled carbon nanotube networks: effect of the location of metal contacts and low-temperature annealing

The ability to control the density of single-walled carbon nanotubes (SWNTs) during the formation of 2D networks allows one to tune the electrical properties of these thin-films from semiconductive to metallic conduction, allowing their use in numerous new materials applications. However, the resistances of such thin-films are generally non-optimal, dominated by the effects of inter-SWNT tunneling junctions, metal/SWNT contacts, sidewall defects, and the presence of residual dopants. These studies provide insight into the relative contributions of these various items to the overall resistance of an SWNT network contacted by Ti electrodes, and ways to reduce these effects via changing the structure of the metal/SWNT contact, and annealing at low temperature. Further, the addition of a mild-acid treatment was found to cause a 13-fold reduction in resistance and much greater reproducibility in inter-network conductivity.

High-Resolution Electrochemical Scanning Tunneling Microscopy (EC-STM) Flow-Cell Studies

Atomic-level studies involving an electrochemical scanning tunneling microscope (EC-STM) flow-cell are presented. Multiple electrochemical atomic layer epitaxy (EC-ALE) cycles of CdTe formation were observed. For a binary compound (i.e., CdTe), an EC-ALE cycle involves exposure of the substrate to a solution of the first precursor (CdSO4), followed by exposure to the second precursor (TeO2), while maintaining potential control. Interleaving blank rinses may also be used, but were omitted in the present studies. To allow the exchange of solutions, the EC-STM cell was modified to allow solution exchange via a single peristaltic pump. A selection valve was used to choose the solution to be introduced into the cell. There is evidence that the growth of the initial layer of CdTe on Au(111), the (√7 × √7)-CdTe monolayer, can be improved in homogeneity and morphology by repeatedly depositing and stripping the Cd atomic layer. Therefore, a new starting cycle, which should improve the quality of deposits formed via EC-ALE, has been developed.

Conversion of metallic single-walled carbon nanotube networks to semiconducting through electrochemical ornamentation.

Field-effect transistors (FETs) that incorporate single-walled carbon nanotube (SWNT) networks experience decreased on-off current ratios (I(on)/I(off)) due to the presence of metallic nanotubes. Herein, we describe a method to increase I(on)/I(off) without the need for either specialized SWNT growth methods or post growth processing steps to remove metallic nanotubes. SWNTs that were grown using conventional arc discharge methods were deposited from aqueous suspension. Then, the SWNTs in the network were decorated with Cu2O nanoparticles that acted as controllable valves that restricted current flow at positive gate voltages. This resulted in an unprecedented reduction in I(off), as the sub-10 nm sized nanoclusters acted as numerous tunable valves, providing greatly improved network sensitivity to gate voltages in the relatively small range of ±10 V, increasing I(on)/I(off) by up to 205-fold. Larger nanoclusters were found to increase the network conductivity but decrease I(on)/I(off). The ability to convert metallic SWNTs to semiconducting without removing them allows for enhanced I(on) and lower noise while still achieving greatly enhanced magnitudes of I(on)/I(off).