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Mark C Hersam

Publications and source records attributed to Mark C Hersam.

14 recordsLinked to original sources
PubMed2006

Syntheses, structures, physical properties, and theoretical studies of CeMxOS (M = Cu, Ag; x approximately 0.8) and CeAgOS.

Black single crystals of the two nonstoichiometric cerium coinage-metal oxysulfide compounds CeCu(x)OS and CeAg(x)OS (x approximately 0.8) have been prepared by the reactions of Ce2S3 and CuO or Ag2O at 1223 or 1173 K, respectively. A black powder sample of CeAgOS has been prepared by the stoichiometric reaction of Ce2S3, CeO2, Ag2S, and Ag at 1073 K. These isostructural materials crystallize in the ZrSiCuAs structure type with two formula units in the tetragonal space group P4/nmm. Refined crystal structure results and chemical analyses provide evidence that the previously known anomalously small unit-cell volume of LnCuOS for Ln = Ce (Ln = rare-earth metal) is the result of Cu vacancies and the concomitant presence of both Ce3+ and Ce4+. Both CeCu(0.8)OS and CeAgOS are paramagnetic with mu(eff) values of 2.13(6) and 2.10(1) mu(B), respectively. CeCu(0.8)OS is a p-type semiconductor with a thermal activation energy Ea = 0.22 eV, sigma(electrical) = 9.8(1) 10(-3) S/cm at 298 K, and an optical band gap Eg < 0.73 eV. CeAgOS has conductivity sigma(conductivity) = 0.16(4) S/cm and an optical band gap Eg = 0.71 eV at 298 K. Theoretical calculations with an on-site Coulomb repulsion parameter indicate that the Ce 4f states are fully spin-polarized and are not localized in CeCuOS, CeCu(0.75)OS, or CeAgOS. Calculated band gaps for CeCu(0.75)OS and CeAgOS are 0.6 and 0.8 eV, respectively.

Journal Article↗
PubMed2006

Microscale features and surface chemical functionality patterned by electron beam lithography: a novel route to poly(dimethylsiloxane) (PDMS) stamp fabrication.

Poly(dimethylsiloxane) (PDMS) has become a ubiquitous material for microcontact printing, yet there are few methods available to pattern a completed PDMS stamp in a single step. It is shown here that electron beam lithography (EBL) is effective in writing patterns directly onto cured PDMS stamps, thus overcoming the need for multiple patterning steps. Not only does this method allow the modification of an existing lithographic pattern, but new 3D features such as cones, pits, and channels can also be fabricated. EBL can also be used to fabricate PDMS masks for photolithography whereby 1:1 pattern transfer into a photoresist is achieved. Additionally, direct EBL writing of surface chemical features has been achieved using a PDMS stamp coated with a self-assembled monolayer. An electrostatic mechanism appears to be operative in the EBL patterning process, as supported by calculations, thermogravimetric analysis, time-of-flight secondary ion mass spectroscopy, optical and atomic force microscopy, and chemical functionalization assays.

Dimethylpolysiloxanes↗
PubMed2006

Imaging of atomic layer deposited (ALD) tungsten monolayers on alpha-TiO2(110) by X-ray standing wave Fourier inversion.

A single atomic layer of tungsten grown by atomic layer deposition (ALD) on a single-crystal rutile TiO2(110) support is studied by the X-ray standing wave (XSW) technique. The surface structural and chemical properties were also examined using atomic force microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. The XSW measured set of hkl Fourier components for the W atomic distribution function are summed together to produce a model-independent 3D map of the W atoms relative to the rutile lattice. The 3D atomic image shows surface tungsten atoms equally occupying the two nonequivalent Ti sites with a slight outward displacement. This corresponds to the atop and bridge sites with respect to the underlying lattice oxygen atoms. These XSW measurements clearly show that ALD conformal layers can be highly coherent with respect to the substrate lattice.

Journal Article↗
PubMed2005

CdO as the archetypical transparent conducting oxide. Systematics of dopant ionic radius and electronic structure effects on charge transport and band structure.

A series of yttrium-doped CdO (CYO) thin films have been grown on both amorphous glass and single-crystal MgO(100) substrates at 410 degrees C by metal-organic chemical vapor deposition (MOCVD), and their phase structure, microstructure, electrical, and optical properties have been investigated. XRD data reveal that all as-deposited CYO thin films are phase-pure and polycrystalline, with features assignable to a cubic CdO-type crystal structure. Epitaxial films grown on single-crystal MgO(100) exhibit biaxial, highly textured microstructures. These as-deposited CYO thin films exhibit excellent optical transparency, with an average transmittance of >80% in the visible range. Y doping widens the optical band gap from 2.86 to 3.27 eV via a Burstein-Moss shift. Room temperature thin film conductivities of 8,540 and 17,800 S/cm on glass and MgO(100), respectively, are obtained at an optimum Y doping level of 1.2-1.3%. Finally, electronic band structure calculations are carried out to systematically compare the structural, electronic, and optical properties of the In-, Sc-, and Y-doped CdO systems. Both experimental and theoretical results reveal that dopant ionic radius and electronic structure have a significant influence on the CdO-based TCO crystal and band structure: (1) lattice parameters contract as a function of dopant ionic radii in the order Y (1.09 A) < In (0.94 A) < Sc (0.89 A); (2) the carrier mobilities and doping efficiencies decrease in the order In > Y > Sc; (3) the dopant d state has substantial influence on the position and width of the s-based conduction band, which ultimately determines the intrinsic charge transport characteristics.

Journal Article↗
PubMed2005

Probing charge transport at the single-molecule level on silicon by using cryogenic ultra-high vacuum scanning tunneling microscopy.

A cryogenic variable-temperature ultra-high vacuum scanning tunneling microscope is used for measuring the electrical properties of isolated cyclopentene molecules adsorbed to the degenerately p-type Si(100)-2x1 surface at a temperature of 80 K. Current-voltage curves taken under these conditions show negative differential resistance at positive sample bias, in agreement with previous observations at room temperature. Because of the enhanced stability of the scanning tunneling microscope at cryogenic temperatures, repeated measurements can be routinely taken over the same molecule. Taking advantage of this improved stability, we show that current-voltage curves on isolated cyclopentene molecules are reproducible and possess negligible hysteresis for a given tip-molecule distance. On the other hand, subsequent measurements with variable tip position show that the negative differential resistance voltage increases with increasing tip-molecule distance. By using a one-dimensional capacitive equivalent circuit and a resonant tunneling model, this behavior can be quantitatively explained, thus providing insight into the electrostatic potential distribution across a semiconductor-molecule-vacuum-metal tunnel junction. This model also provides a quantitative estimate for the alignment of the highest occupied molecular orbital of cyclopentene with respect to the Fermi level of the silicon substrate, thus suggesting that this experimental approach can be used for performing chemical spectroscopy at the single-molecule level on semiconductor surfaces. Overall, these results serve as the basis for a series of design rules that can be applied to silicon-based molecular electronic devices.

Electron Transport↗
PubMed2005

Encapsulation of carbon nanotubes by self-assembling peptide amphiphiles.

We demonstrate the dispersion and noncovalent functionalization of carbon nanotubes in water using peptide amphiphiles each consisting of a short hydrophobic alkyl tail coupled to a more hydrophilic peptide sequence. The assembly of peptide amphiphile molecules on the surfaces of carbon nanotubes adds biofunctionality to these one-dimensional conductors and simultaneously eliminates the hydrophobic nanotube-water interface, thus dispersing them in the aqueous medium. This should occur without the degradation of their structural, electronic, and optical properties caused by covalent functionalization and without the need for specific peptide sequences designed to bind with nanotube surfaces. The encapsulation by peptide amphiphiles is confirmed using transmission electron microscopy and optical absorbance spectroscopy and may have significant future applications in biosensing or medicine.

Microscopy, Electron↗
PubMed2005

Adhesive transfer of thin viscoelastic films.

Micellar suspensions of acrylic diblock copolymers are excellent model materials for studying the adhesive transfer of viscoelastic solids. The micellar structure is maintained in films with a variety of thicknesses, giving films with a well-defined structure and viscoelastic character. Thin films were cast onto elastomeric silicone substrates from micellar suspensions in butanol, and the adhesive interactions between these coated elastomeric substrates and a rigid indenter were quantified. By controlling the adhesive properties of the film/indenter and film/substrate interfaces we were able to obtain very clean transfer of the film from the substrate to the portion of the glass indenter with which the film was in contact. Adhesive failure at the film/substrate interface occurs when the film/indenter interface is able to support an applied energy release rate that is sufficient to result in cavity nucleation at the film/substrate interface. Cavity formation is rapidly followed by delamination of the entire region under the indenter. The final stage in the transfer process involves the failure of the film that bridges the indenter and the elastomeric substrate. This film is remarkably robust and is extended to three times its original width prior to failure. Failure of this film occurs at the periphery of the indenter, giving a transferred film that conforms to the original contact area between the indenter and the coated substrate.

Journal Article↗
PubMed2005

Conductive atomic force microscope nanopatterning of hydrogen-passivated silicon in inert organic solvents.

Ambient liquid phase atomic force microscope (AFM) techniques for nanopatterning organic molecules on silicon through direct Si-C bonds rely on reactions that are in direct competition with spurious oxidation. We study the effectiveness of an inert hydrophobic organic solvent at suppressing oxidation of hydrogen-passivated silicon under ambient conditions. Nanometer scale features were fabricated on an H:Si(111) substrate using a conductive AFM in hexadecane. The patterned features show chemical and kinetic behavior consistent with field induced oxidation (FIO) in air. The mechanism for FIO in hexadecane is discussed.

Journal Article↗
PubMed2005

Enrichment of single-walled carbon nanotubes by diameter in density gradients.

The bulk enrichment and separation of single-walled carbon nanotubes (SWNTs) by diameter has been achieved through ultracentrifugation of DNA-wrapped SWNTs in aqueous density gradients. The separation is identified by the visual formation of colored bands of SWNTs in the density range of 1.11-1.17 g cm(-3). The optical absorbance spectra of the separated SWNTs indicate that SWNTs of decreasing diameter are increasingly more buoyant. This nondestructive and scalable separation strategy is expected to impact the fields of molecular electronics, optoelectronics, and sensing where SWNTs of a monodisperse band gap are essential.

Centrifugation, Density Gradient↗
PubMed2004

X-ray studies of self-assembled organic monolayers grown on hydrogen-terminated Si(111).

The structure of self-assembled monolayers (SAMs) of undecylenic acid methyl ester (SAM-1) and undec-10-enoic acid 2-bromo-ethyl ester (SAM-2) grown on hydrogen-passivated Si(111) were studied by X-ray reflectivity (XRR), X-ray standing waves (XSW), X-ray fluorescence (XRF), atomic force microscopy, and X-ray photoelectron spectroscopy (XPS). The two different SAMs were grown by immersion of H-Si(111) substrates into the two different concentrated esters. UV irradiation during immersion was used to create Si dangling bond sites that act as initiators of the surface free-radical addition process that leads to film growth. The XRR structural analysis reveals that the molecules of SAM-1 and SAM-2 respectively have area densities corresponding to 50% and 57% of the density of Si(111) surface dangling bonds and produce films with less than 4 angstroms root-mean-square roughness that have layer thicknesses of 12.2 and 13.2 angstroms. Considering the molecular lengths, these thicknesses correspond to a 38 degrees and 23 degrees tilt angle for the respective molecules. For SAM-2/Si(111) samples, XRF analysis reveals a 0.58 monolayer (ML) Br total coverage. Single-crystal Bragg diffraction XSW analysis reveals (unexpectedly) that 0.48 ML of these Br atoms are at a Si(111) lattice position height that is identical to the T1 site that was previously found by XSW analysis for Br adsorbed onto Si(111) from a methanol solution and from ultrahigh vacuum. From the combined XPS, XRR, XRF, and XSW evidence, it is concluded that Br abstraction by reactive surface dangling bonds competes with olefin addition to the surface.

Hydrogen↗
PubMed2004

Application of scanning probe microscopy to the characterization and fabrication of hybrid nanomaterials.

Scanning probe microscopy (SPM) is a widely used experimental technique for characterizing and fabricating nanostructures on surfaces. In particular, due to its ability to spatially map variations in materials properties with nanometer spatial resolution, SPM is particularly well suited to probe the subcomponents and interfaces of hybrid nanomaterials, i.e., materials that are made up of distinct nanometer scale components with distinguishable properties. In addition, the interaction of the SPM tip with materials can be intentionally tuned such that local surface modification is achieved. In this manner, hybrid nanostructures can also be fabricated on solid substrates using SPM. This report reviews recent developments in the characterization and fabrication of hybrid nanomaterials with SPM. Specific attention is given to nanomaterials that consist of both organic and inorganic components including individual biomolecules mounted on inorganic substrates. SPM techniques that are particularly well suited for characterizing the mechanical and electrical properties of such hybrid systems in atmospheric pressure environments are highlighted, and specific illustrative examples are provided. This review concludes with a brief discussion of the remaining challenges and promising future prospects for this field.

Animals↗
PubMed2003

Molecular electronics on silicon: an ultrahigh vacuum scanning tunneling microscopy study.

An ultrahigh vacuum scanning tunneling microscope (STM) was employed as a tool to characterize two distinct molecular electronic strategies on the Si(100) surface. Initially, the self-directed growth of one-dimensional styrene molecular chains on hydrogen-passivated Si(100) was investigated. High-resolution empty states imaging of these styrene nanostructures confirms alignment of phenyl groups along the chain. However, attempts at STM charge transport measurements were limited by tip induced desorption of styrene molecules. Consequently, an alternative oxygen radical chemistry was also investigated. In particular, the chemical adsorption of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) onto clean Si(100) leads to the formation of an exceptionally stable silicon-oxygen bond that can withstand high bias charge transport measurements up to +/- 5 volts. Direct charge transport measurements through individual TEMPO molecules on degenerately n-type doped Si(100) reveal room temperature negative differential resistance behavior for negative sample biases exceeding - 3 volts.

Crystallography↗