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Abstract:
The exceptional properties of graphene enable applications in electronics, optoelectronics, energy storage, and structural composites. Here we demonstrate a 3D printable graphene (3DG) composite consisting of majority graphene and minority polylactide-co-glycolide, a biocompatible elastomer, 3D-printed from a liquid ink. This ink can be utilized under ambient conditions via extrusion-based 3D printing to create graphene structures with features as small as 100 μm composed of as few as two layers (<300 μm thick object) or many hundreds of layers (>10 cm thick object). The resulting 3DG material is mechanically robust and flexible while retaining electrical conductivities greater than 800 S/m, an order of magnitude increase over previously reported 3D-printed carbon materials. In vitro experiments in simple growth medium, in the absence of neurogenic stimuli, reveal that 3DG supports human mesenchymal stem cell (hMSC) adhesion, viability, proliferation, and neurogenic differentiation with significant upregulation of glial and neuronal genes. This coincides with hMSCs adopting highly elongated morphologies with features similar to axons and presynaptic terminals. In vivo experiments indicate that 3DG has promising biocompatibility over the course of at least 30 days. Surgical tests using a human cadaver nerve model also illustrate that 3DG has exceptional handling characteristics and can be intraoperatively manipulated and applied to fine surgical procedures. With this unique set of properties, combined with ease of fabrication, 3DG could be applied toward the design and fabrication of a wide range of functional electronic, biological, and bioelectronic medical and nonmedical devices.

Citation: A. E. Jakus, E. B. Secor, A. L. Rutz, S. W. Jordan, M. C. Hersam, and R. N. Shah, “Three-dimensional printing of high-content graphene scaffolds for electronic and biomedical applications,” ACS Nano, 9, 4636 (2015).
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Abstract:
A new type of carbon charge-transfer magnet, consisting of a fullerene acceptor and single-walled carbon nanotube donor, is demonstrated, which exhibits room temperature ferromagnetism and magnetoelectric (ME) coupling. In addition, external stimuli (electric/magnetic/elastic field) and the concentration of a nanocarbon complex enable the tunabilities of the magnetization and ME coupling due to the control of the charge transfer.

Citation: W. Qin, M. Gong, X. Chen, T. A. Shastry, R. Sakidja, G. Yuan, M. C. Hersam, M. Wuttig, and S. Ren, “Multiferroicity of carbon-based charge transfer magnets,” Adv. Mater., 27, 734 (2015).
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Abstract:
The trade-off between light absorption and exciton diffusion length must be addressed before widespread deployment of organic photovoltaics can be realized. Optical transfer matrix modeling is used in inverted, high-efficiency organic photovoltaics, employing a poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethyl-hexyl)carbonyl] thieno[3,4-b]thiophene¬diyl]] (PTB7):[6,6]-phenyl C71butyric acid methyl-ester (PC71BM) active layer to spectrally sculpt absorption enhancement by tuning the layer thicknesses of both the photoactive layer and the ZnO interfacial layer (IFL).

Citation: S. Loser, B. Valle, K. A. Luck, C. K. Song, G. Ogien, M. C. Hersam, K. D. Singer, and T. J. Marks, “High-efficiency inverted polymer photovoltaics via spectrally tuned absorption enhancement,” Adv. Energy Mater., 4, 1301938 (2014).
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Abstract:
Recent advances in semiconductor performance made possible by organic π-electron molecules, carbon-based nanomaterials, and metal oxides have been a central scientific and technological research focus over the past decade in the quest for flexible and transparent electronic products. However, advances in semiconductor materials require corresponding advances in compatible gate dielectric materials, which must exhibit excellent electrical properties such as large capacitance, high breakdown strength, low leakage current density, and mechanical flexibility on arbitrary substrates. Historically, conventional silicon dioxide (SiO2) has dominated electronics as the preferred gate dielectric material in complementary metal oxide semiconductor (CMOS) integrated transistor circuitry. However, it does not satisfy many of the performance requirements for the aforementioned semiconductors due to its relatively low dielectric constant and intransigent processability. High-k inorganics such as hafnium dioxide (HfO2) or zirconium dioxide (ZrO2) offer some increases in performance, but scientists have great difficulty depositing these materials as smooth films at temperatures compatible with flexible plastic substrates. While various organic polymers are accessible via chemical synthesis and readily form films from solution, they typically exhibit low capacitances, and the corresponding transistors operate at unacceptably high voltages. More recently, researchers have combined the favorable properties of high-k metal oxides and π-electron organics to form processable, structurally well-defined, and robust self-assembled multilayer nanodielectrics, which enable high-performance transistors with a wide variety of unconventional semiconductors.

In this Account, we review recent advances in organic–inorganic hybrid gate dielectrics, fabricated by multilayer self-assembly, and their remarkable synergy with unconventional semiconductors. We first discuss the principals and functional importance of gate dielectric materials in thin-film transistor (TFT) operation. Next, we describe the design, fabrication, properties, and applications of solution-deposited multilayer organic–inorganic hybrid gate dielectrics, using self-assembly techniques, which provide bonding between the organic and inorganic layers. Finally, we discuss approaches for preparing analogous hybrid multilayers by vapor-phase growth and discuss the properties of these materials.

Citation: Y.-G. Ha, K. Everaerts, M. C. Hersam, and T. J. Marks, “Hybrid gate dielectric materials for unconventional electronic circuitry,” Acc. Chem. Res., 47, 1019 (2014).
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Abstract:
Molecular-scale control over the integration of disparate materials on graphene is a critical step in the development of graphene-based electronics and sensors. Here, we report that self-assembled monolayers of 10,12-pentacosadiynoic acid (PCDA) on epitaxial graphene can be used to template the reaction and directed growth of atomic layer deposited (ALD) oxide nanostructures with sub-10 nm lateral resolution. PCDA spontaneously assembles into well-ordered domains consisting of one-dimensional molecular chains that coat the entire graphene surface in a manner consistent with the symmetry of the underlying graphene lattice. Subsequently, zinc oxide and alumina ALD precursors are shown to preferentially react with the functional moieties of PCDA, resulting in templated oxide nanostructures. The retention of the original one-dimensional molecular ordering following ALD is dependent on the chemical reaction pathway and the stability of the monolayer, which can be enhanced via ultraviolet-induced molecular cross-linking.

Citation: J. M. P. Alaboson, C.-H. Sham, S. Kewalramani, J. D. Emery, J. E. Johns, A. Deshpande, T. Chien, M. J. Bedzyk, J. W. Elam, M. J. Pellin, and M. C. Hersam, “Templating sub-10 nm atomic layer deposited oxide nanostructures on graphene via one-dimensional organic self-assembled monolayers,” Nano Lett., 13, 5763 (2013).
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Abstract:
High-performance broad-spectrum nanocarbon bulk-heterojunction photovoltaic photodetectors are reported. These reported photodetectors consist of a semiconducting single-walled carbon nanotube (s-SWCNT) and a PC71BM blended active layer. Magnetic-field effects and the chirality of the s-SWCNTs play an important role in controlling the photoresponse time and photocurrent improvement.

Citation: Y. Xie, M. Gong, T. A. Shastry, J. Lohrman, M. C. Hersam, and S. Ren, “Broad-spectral-response nanocarbon bulk-heterojunction excitonic photodetectors,” Adv. Mater., 25, 3433 (2013).
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Abstract:
Recent years have seen a proliferation of scanning probe microscopy (SPM) techniques that can probe and manipulate a diverse range of materials and devices. In particular, SPM methods that employ a conductive tip are well suited for probing electronic and electrochemical phenomena of direct relevance to electronic and energy technologies. Conductive SPM is also a versatile nanofabrication tool, which can create nearly arbitrary nanopatterns of oxide, metals, and organics on solid substrates. In this Feature Article, we provide an overview of recent conductive SPM work from our laboratory regarding the characterization and nanopatterning of electronic and energy materials. The discussion begins by describing the methodologies used to characterize organic photovoltaics and transparent conducting oxides. We then illustrate how different SPM techniques are applied to the more complex electrochemical environments presented by Li-ion batteries and other electrochemical systems. Lastly, the use of conductive atomic force microscopy to probe and nanopattern electronically inhomogeneous substrates, such as epitaxial graphene layers on silicon carbide, is presented.

Citation: A. L. Lipson and M. C. Hersam, “Conductive Scanning Probe Characterization and Nanopatterning of Electronic and Energy Materials,” J. Phys Chem. C, 117, 7953 (2013).
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Abstract:
Multiple vibrational modes have been observed for copper phthalocyanine (CuPc) adlayers on Ag(111) using ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS). Several important new experimental features are introduced in this work that significantly advance the state-of-the-art in UHV-TERS. These include (1) concurrent sub-nm molecular resolution STM imaging using Ag tips with laser illumination of the tip–sample junction, (2) laser focusing and Raman collection optics that are external to the UHV-STM that has two cryoshrouds for future low temperature experiments, and (3) all sample preparation steps are carried out in UHV to minimize contamination and maximize spatial resolution. Using this apparatus we have been able to demonstrate a TERS enhancement factor of 7.1 × 105. Further, density-functional theory calculations have been carried out that allow quantitative identification of eight different vibrational modes in the TER spectra. The combination of molecular-resolution UHV-STM imaging with the detailed chemical information content of UHV-TERS allows the interactions between large polyatomic molecular adsorbates and specific binding sites on solid surfaces to be probed with unprecedented spatial and spectroscopic resolution.

Citation: N. Jiang, E. T. Foley, J. M. Klingsporn, M. D. Sonntag, N. A. Valley, J. A. Dieringer, T. Seideman, G. C. Schatz, M. C. Hersam, and R. P. Van Duyne, “Observation of multiple vibrational modes in ultrahigh vacuum tip-enhanced Raman spectroscopy combined with molecular-resolution scanning tunneling microscopy,” Nano Letters, 12, 5061 (2012).
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Abstract:
With its exceptional charge transport properties, graphene has emerged as a potential replace- ment material in the electronics industry. For these applications, much effort has been devoted towards the synthesis of large defect-free graphene sheets. However, recent developments have enabled the efficient production of micrometer- and nanometer-sized graphene sheets in the solution phase. These suspensions have stimulated the development of novel materials and devices that more fully exploit the tunability and large specific surface area of pris- tine graphene. This review highlights advances in the under- standing of the defect structure and properties of as-produced graphene as well as strategies for its chemical selection and modification that facilitates its use in functional materials and devices.

Citation: Y. T. Liang and M. C. Hersam, "Towards Rationally Designed Graphene-Based Materials and Devices," Macromolecular Chemistry and Physics, 213, 1091 (2011).
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Abstract:
Scanning ion conductance microscopy imaging of battery electrodes, using the geometry shown in the figure, is a tool for in situ nanoscale mapping of surface topography and local ion current. Images of silicon and tin electrodes show that the combination of topography and ion current provides insight into the local electrochemical phenomena that govern the operation of lithium ion batteries.

Citation: A. L. Lipson, R. S. Ginder, and M. C. Hersam, "Nanoscale In Situ Characterization of Li-ion Battery Electrochemistry Via Scanning Ion Conductance Microscopy," Advanced Materials, 23, 5613 (2011).
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Abstract:
Single-walled carbon nanotubes (SWNTs) sorted by electronic type are employed as organic photovoltaic device anodes. Metal-enriched SWNT films yield device efficiencies that are fifty times greater than their semiconducting counterparts. Through sheet resistance, UV-vis-NIR optical absorbance, and X-ray photoelectron spectroscopy measurements, the OPV charge blocking layer PEDOT:PSS is found to reverse the original chemical doping of the SWNT films. The relative insensitivity of metallic SWNTs to chemical doping thus explains the improved performance of metal-enriched SWNT films as OPV anodes.

Citation: T. P. Tyler, R. E. Brock, H. J. Karmel, T. J. Marks, and M. C. Hersam, "Electronically Monodisperse Single-Walled Carbon Nanotube Thin Films as Transparent Conducting Anodes in Organic Photovoltaic Devices," Advanced Energy Materials, 1, 785 (2011).
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Abstract:
We describe how iterative, orthogonal density gradient ultracentrifugation (DGU) separations can be used to produce nearly single-chirality (6,5) SWNTs. SWNT network transistors made from these highly pure, ≥98% semiconducting SWNTs simultaneously exhibit high on/off ratios, mobilities, and on-state conductances, suggesting their future application in integrated circuits and near-infrared optoelectronic light emitters and photodetectors.

Citation: A. A. Green and M. C. Hersam, "Nearly single-chirality single-walled carbon nanotubes produced via orthogonal iterative density gradient ultracentrifugation," Advanced Materials, 23, 2185 (2011).
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Abstract:
Nanostructured metal films have been widely studied for their roles in sensing, catalysis, and energy storage. In this work, the synthesis of compositionally controlled and nanostructured Pt/Ir films by atomic layer deposition (ALD) into porous anodized aluminum oxide templates is demonstrated. Templated ALD provides advantages over alternative synthesis techniques, including improved film uniformity and conformality as well as atomic-scale control over morphology and composition. Nanostructured Pt ALD films are demonstrated with morphological control provided by the Pt precursor exposure time and the number of ALD cycles. With these approaches, Pt films with enhanced surface areas, as characterized by roughness factors as large as 310, are reproducibly synthesized. Additionally, nanostructured PtIr alloy films of controlled composition and morphology are demonstrated by templated ALD, with compositions varying systematically from pure Pt to pure Ir. Lastly, the application of nanostructured Pt films to electrochemical sensing applications is demonstrated by the non-enzymatic sensing of glucose.

Citation: D. J. Comstock, S. T. Christensen, J. W. Elam, M. J. Pellin, and M. C. Hersam, "Tuning the composition and nanostructure of Pt/Ir films via anodized aluminum oxide templated atomic layer deposition," Adv. Funct. Mater., 20, 3099 (2010).
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Abstract:
Scanning tunneling microscopy (STM), atomic force microscopy (AFM), lateral force microscopy (LFM), and conductive AFM (cAFM) are employed to characterize epitaxial graphene on SiC(0001). Of particular interest are substrates that possess single-layer and bilayer graphene domains, which form during thermal decomposition of silicon from SiC(0001). Since these samples are often partially graphitized, characterization techniques are needed that can distinguish domains of epitaxial graphene from the adjacent (6√3×6√3)R30° reconstructed SiC(0001) surface. The relative merits of STM, AFM, LFM, and cAFM for this purpose are outlined, thus providing nanometer-scale strategies for identifying and characterizing epitaxial graphene.

Citation: J. A. Kellar, J. M. P. Alaboson, Q. H. Wang, and M. C. Hersam, "Identifying and characterizing epitaxial graphene domains on partially graphitized SiC(0001) surfaces using scanning probe microscopy," Appl. Phys. Lett., 96, 143103 (2010).
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Abstract:
With the recent burst of activity surrounding the solution-phase production of graphene, comparatively little progress has been made toward the generation of graphene dispersions with tailored thickness, lateral area, and shape. The polydispersity of graphene dispersions, however, can lead to unpredictable or nonideal behavior once they are incorporated into devices since the properties of graphene vary as a function of its structural parameters. In this brief perspective, we overview the problem of graphene polydispersity, the production of graphene dispersions, and the methods under development to produce dispersions of monodisperse graphene.

Citation: A. A. Green and M. C. Hersam, "Emerging methods for producing monodisperse graphene solutions," J. Phys. Chem. Lett., 1, 544 (2010).
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Abstract:
Graphene, a two-dimensional sheet of carbon atoms, is a promising material for next-generation technology because of its advantageous electronic properties, such as extremely high carrier mobilities. However, chemical functionalization schemes are needed to integrate graphene with the diverse range of materials required for device applications. In this paper, we report self-assembled monolayers of the molecular semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) formed on epitaxial graphene grown on the SiC(0001) surface. The molecules possess long-range order with a herringbone arrangement, as shown by ultra-high vacuum scanning tunnelling microscopy at room temperature. The molecular ordering is unperturbed by defects in the epitaxial graphene or atomic steps in the underlying SiC surface. Scanning tunnelling spectra of the PTCDA monolayer show distinct features that are not observed on pristine graphene. The demonstration of robust, uniform organic functionalization of epitaxial graphene presents opportunities for graphene-based molecular electronics and sensors.

Citation: Q. H. Wang and M. C. Hersam, "Room-temperature molecular-resolution characterization of self-assembled organic monolayers on epitaxial graphene," Nature Chemistry 1, 206 (2009).
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Abstract:
With an eye toward using surface morphology to enhance heterogeneous catalysis, Pt nanoparticles are grown by atomic layer deposition (ALD) on the surfaces of SrTiO3 nanocubes. The size, dispersion, and chemical state of the Pt nanoparticles are controlled by the number of ALD growth cycles. The SrTiO3 nanocubes average 60 nm on a side with {001} faces. The Pt loading increases linearly with Pt ALD cycles to a value of 1.1 × 10-6 g cm-2 after five cycles. Scanning electron microscopy images reveal discrete, well-dispersed Pt nanoparticles. Small- and wide-angle X-ray scattering show that the Pt nanoparticle spacing and size increase as the number of ALD cycles increases. X-ray absorption spectroscopy shows a progression from platinum(II) oxide to metallic platinum and a decrease in PtO bonding with an increase in PtPt bonding as the number of ALD cycles increases.

Citation: S. T. Christensen, J. W. Elam, F. A. Rabuffetti, Q. Ma, S. J. Weigand, B. Lee, S. Seifert, P. C. Stair, K. R. Poeppelmeier, M. C. Hersam, and M. J. Bedzyk, "Controlled growth of platinum nanoparticles on strontium titanate nanocubes by atomic layer deposition," Small 5, 750 (2009).
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Abstract:
Embedded, optically transparent, electrically conducting oxide nanowires, and other patterns are written on highly resistive transparent metal oxide thin films with nanoscale spatial control using focused ion beam implantation. The resulting transparent conducting oxide features are 110-160 nm wide, 7 nm deep, and are theoretically limitless in length, connectivity, and shape.

Citation: N. E. Sosa, J. Liu, C. Chen, T. J. Marks, and M. C. Hersam, "Nanoscale writing of transparent conducting oxide features with a focused ion beam," Advanced Materials 21, 721 (2009).
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Abstract:
Current methods of synthesizing single-walled carbon nanotubes (SWNTs) result in racemic mixtures that have impeded the study of left- and right-handed SWNTs. Here we present a method of isolating different SWNT enantiomers using density gradient ultracentrifugation. Enantiomer separation is enabled by the chiral surfactant sodium cholate, which discriminates between left- and right-handed SWNTs and thus induces subtle differences in their buoyant densities. This sorting strategy can be employed for simultaneous enrichment by handedness and roll-up vector of SWNTs having diameters ranging from 0.7 to 1.5 nm. In addition, circular dichroism of enantiomer refined samples enables identification of high-energy optical transitions in SWNTs.

Citation: A. A. Green, M. C. Duch, and M. C. Hersam, "Isolation of single-walled carbon nanotube enantiomers by density differentiation," Nano Research 2, 69 (2009).
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Abstract:
Carbon nanotubes consist of one or more concentric graphene cylinders and are under investigation for a variety of applications that make use of their excellent thermal, mechanical, electronic and optical properties. Double-wall nanotubes are ideal systems for studying the interwall interactions influencing the properties of nanotubes with two or more walls. However, current techniques to synthesize double-wall nanotubes produce unwanted single- and multiwall nanotubes. Here, we show how density gradient ultracentrifugation can be used to separate double-wall nanotubes from mixtures of single- and multiwall nanotubes through differences in their buoyant density. This technique results in samples that are highly enriched in either single- or double-wall nanotubes of similar outer wall diameter, with the double-wall nanotubes being, on average, 44% longer than the single-wall nanotubes. The longer average length of the double-wall nanotubes provides distinct advantages when they are used in transparent conductors.

Citation: A. A. Green and M. C. Hersam, "Processing and properties of highly enriched double-wall carbon nanotubes," Nature Nanotechnology 4, 64 (2009).
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Abstract:
The hydrodynamic properties of surfactant encapsulated single-walled carbon nanotubes (SWNTs) have been characterized by optically measuring their spatial and temporal redistribution in situ in an analytical ultracentrifuge. The measured redistribution profiles are fit to the Lamm equation, thus determining the sedimentation, diffusion, and hydrodynamic frictional coefficients of the surfactant encapsulated SWNTs. For sodium cholate encapsulated SWNTs, we demonstrate that the technique of analytical ultracentrifugation can be utilized to determine the linear packing density of surfactant molecules along the length of the SWNTs, 3.6 ± 0.8 nm−1, and the anhydrous molar volume of the surfactant molecules on the SWNT surfaces, 270 ± 20 cm3 mol−1. Additionally, analytical ultracentrifugation is used to measure and compare the sedimentation rates of bundled and isolated carbon nanotubes. This study should serve as a guide for designing centrifuge-based processing procedures for preparing samples of SWNTs for a wide variety of applications and studies. Additionally, the results obtained here should aid in understanding the hydrodynamic properties of SWNTs and the interactions between SWNTs and surfactants in aqueous solution.

Citation: M. S. Arnold, J. Suntivich, S. I. Stupp, and M. C. Hersam, "Hydrodynamic characterization of surfactant encapsulated carbon nanotubes using an analytical ultracentrifuge," ACS Nano 2, 2291 (2008).
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Abstract:
Self-assembled Ga nanoclusters form a well-ordered array on the Si(111)-7 × 7 surface. Scanning tunneling microscopy and spectroscopy map the electronic structure of these arrays at the atomic scale. In the image, the local density of states is plotted as a color map over the topography and shows a continuous 2D network of increased differential tunneling conductance connecting the nanoclusters.

Citation: Q. H. Wang and M. C. Hersam, "Atomically resolved charge redistribution for Ga nanocluster arrays on the Si(111)-7x7 surface," Small 4, 915 (2008).
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Abstract:
Room-temperature ultra-high vacuum (UHV) scanning tunnelling microscopy (STM) has been employed to investigate free radical chemistry on the Si(111)-7 × 7 surface with atomic-scale spatial resolution. In particular, due to its single-site binding mechanism and extensive previous study on the Si(100)-2 × 1 surface, the nitroxyl free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was explored. UHV STM imaging of isolated molecules revealed that TEMPO covalently reacts with adatom dangling bonds with high affinity. By monitoring TEMPO adsorption as a function of surface coverage, TEMPO was also found to preferentially bind to centre adatom sites at the initial stages of adsorption. On the other hand, as the surface coverage increased, TEMPO molecules adsorbed to centre adatoms and corner adatoms approached a ratio of 1:1. At all surface coverage levels, TEMPO showed minimal preference for binding to either the faulted or unfaulted half of the unit cell. Consequently, upon saturation, the TEMPO adlayer exhibited long-range ordering and preserved the underlying 7 × 7 surface reconstruction. This study provides fundamental insight into free radical surface chemistry and suggests a direct pathway for forming nearly perfectly ordered organic adlayers on the Si(111)-7 × 7 surface.

Citation: N. P. Guisinger, S. P. Elder, N. L. Yoder, and M. C. Hersam, "Ultra-high vacuum scanning tunneling microscopy investigation of free radical adsorption to the Si(111)-7x7 surface," Nanotechnology 18, 044011 (2007).
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Abstract:
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.

Citation: M. T. Russell, L. S. C. Pingree, M. C. Hersam, and T. J. Marks, "Micro-scale features and surface chemical functionality patterned by electron beam lithography. A novel route to poly(dimethylsiloxane) (PDMS) stamp fabrication," Langmuir 22, 6712 (2006).
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Abstract:
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)-2×1 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.

Citation: N. P. Guisinger, N. L. Yoder, and M. C. Hersam, "Probing charge transport at the single molecule level on silicon by using cryogenic ultra-high vacuum scanning tunneling microscopy," Proceedings of the National Academy of Sciences USA 102, 8838 (2005).
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Abstract:
Ultrahigh vacuum scanning tunneling microscopy is employed for the nanofabrication and characterization of atomically registered heteromolecular organosilicon nanostructures at room temperature. In the first fabrication step, feedback controlled lithography (FCL) is used to pattern individual 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radical molecules at opposite ends of the same dimer row on the Si(100)-2×1:H surface. In atomic registration with the first pattern, FCL is subsequently applied for the removal of a single hydrogen atom. The resulting dangling bond templates the spontaneous growth of a styrene chain that is oriented along the underlying dimer row. The styrene chain growth is bounded by the originally patterned TEMPO molecules, thus resulting in a heteromolecular organosilicon nanostructure. The demonstration of multistep FCL suggests that this approach can be widely used for fundamental studies and fabricating prototype devices that require atomically registered organic molecules mounted on silicon surfaces.

Citation: R. Basu, N. P. Guisinger, M. E. Greene, and M. C. Hersam, "Room temperature nanofabrication of atomically registered heteromolecular organosilicon nanostructures using multistep feedback controlled lithography," Applied Physics Letters 85, 2619 (2004).
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Abstract:
The ultrahigh vacuum scanning tunnelling microscope was used to probe charge transport through two different organic monolayers adsorbed on the Si(100) substrate at room temperature. I–V measurements were taken on monolayers of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and cyclopentene for degenerately doped n-type and p-type substrates. Initial I–V measurements for transport through the TEMPO monolayer exhibited a suppression of negative differential resistance (NDR) relative to previously reported charge transport through isolated molecules. I–V measurements were also performed on isolated cyclopentene molecules and cyclopentene monolayers. Similarly to TEMPO monolayers, the cyclopentene monolayers exhibited attenuated NDR behaviour relative to isolated molecules. The observed NDR suppression suggests that the high packing density of organic monolayers influences charge transport through molecule–semiconductor junctions.

Citation: N. P. Guisinger, R. Basu, M. E. Greene, A. S. Baluch, and M. C. Hersam, "Observed suppression of room temperature negative differential resistance in organic monolayers on Si(100)," Nanotechnology 15, S452 (2004).
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Abstract:
Room temperature negative differential resistance (NDR) has been measured through individual organic molecules on degenerately doped Si(100) surfaces using ultrahigh vacuum scanning tunneling microscopy (STM). For styrene molecules on n-type Si(100), NDR is observed only for negative sample bias because positive sample bias leads to electron stimulated desorption. By replacing styrene with a saturated organic molecule (2,2,6,6-tetramethyl-1-piperidinyloxy), electron stimulated desorption is not observed at either bias polarity. In this case, NDR is observed only for negative sample bias on n-type Si(100) and for positive sample bias on p-type Si(100). This unique behavior is consistent with a resonant tunneling mechanism via molecular orbitals and opens new possibilities for silicon-based molecular electronic devices and chemical identification with STM at the single-molecule level.

Citation: N. P. Guisinger, M. E. Greene, R. Basu, A. S. Baluch, and M. C. Hersam, "Room temperature negative differential resistance through individual molecules on silicon surfaces," Nano Lett. 4, 55 (2004).

The Hersam Group studies, develops, and manipulates hybrid hard and soft nanoscale materials for applications in information technology, biotechnology, nanotechnology, and alternative energy.

Research Highlights



Gate-tunable molybdenum disulfide memristers

Single layer molybdenum disulfide shows memristive behavior emerging from grain boundary resistance that can be modulated by an applied gate potential.
Nature Nanotechnology, 2015

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Solution-processed anti-ambipolar p-n junctions

Large area heterojunctions of carbon nanotubes and indium gallium zinc oxide demonstrate anti-ambipolar behavior that can be utilized in integrated circuits.
Nano Letters, 2015

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Air-stable black phosphorus

A coating of aluminum oxide prevents the rapid degradiation of black phosphorus under ambient conditions, enabling stable and high-performance transistors.
Nano Letters, 2014

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Sorting two-dimensional metal dichalcogenides by thickness

Density gradient ultracentrifugation can sort molybdenum disulfide by thickness when the flakes are dispersed using block copolymers.
Nature Communications, 2014

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Efficient, stable carbon nanotube solar cells

A polychiral distribution of carbon nanotubes enables world-record carbon nanotube solar cell efficiency.
Nano Letters, 2014

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Gravure Printing of Graphene

Processing optimization of graphene inks enables their use in gravure printing of flexible printed electronics.
Advanced Materials, 2014

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