Surface potentials and layer charge distributions in few-layer graphene
S. S. Datta, D. R. Strachan, E. J. Mele, A. T. Johnson, Nano Letters 9, 7 (2009).
Also available at arxiv:0807.1650.

This work was featured on the cover of the January 2009 issue of Nano Letters.

This work was featured on Penn communications, AAAS EurekAlert, Astronomy Now, Azomaterials, Azonano, Bio-Medicine, ElectronicsWeb, Nanotechnology Now, Nanotechnology Today, ScienceDaily, PhysOrg, e! Science News, Nanotechwire, Nanowerk, and Daily Science News.

Graphene-derived nanomaterials are emerging as ideal candidates for postsilicon electronics devices, with potential applications as atomically thin transistors, sensors, and other nanoelectronic devices incorporating quantum size effects. For this to happen, a number of questions need to be addressed. For example, how does the substrate it sits on affect the electronic properties of few-layer graphene (FLG)? Do charge exchange processes occur at this interface; and if so, how do the doped charges spread themselves out over the different graphene layers in a FLG film? Elucidating the electronic interaction between an insulating substrate and FLG films is crucial for device applications. Here we use a form of scanning probe microscopy, electrostatic force microscopy (EFM), to probe the electrostatic interactions within FLG samples on oxidized silicon substrates. Our measurements reveal behavior in sharp contrast with that expected for conventional conducting or semiconducting films; rather, it derives from unique aspects of charge screening by graphene's relativistic low energy carriers. We propose a nonlinear Thomas-Fermi theory for the FLG charge carriers and find excellent quantitative agreement with the data. Our EFM measurements also reveal previously unseen electronic perturbations extended along crystallographic directions of structurally disordered FLGs, likely resulting from long-range atomic defects. These results have important implications for graphene nanoelectronics and provide a powerful framework by which key properties can be further investigated.

Crystallographic etching of few-layer graphene
S. S. Datta, D. R. Strachan, S. M. Khamis, A. T. Johnson, Nano Letters 8, 1912 (2008).
Also available at arxiv:0806.3965.

This work was featured in Nature Nanotechnology, 4 July 2008 as a research highlight. It was also featured on Penn communications, Nanotella, AAAS EurekAlert, Azomaterials, Azonano, Bio-Medicine, ScienceDaily, PhysOrg, e! Science News, Nanotechnology Now, Nanotechwire, Photonics Online, Science Codex, Semiconductor International, and Daily Science News.

While large-area FLG flakes continue to attract significant attention because of their remarkable electronic properties, other sample geometries could be very exciting as well. Of particular interest would be the construction of graphene nanoribbons, in which charge carriers are confined in the lateral dimension whereby the electronic properties are controlled by the width and specific crystallographic orientation of the ribbon. Very little progress has been made in using current lithographic techniques to fabricate graphene nanoribbons with crystallographic edges - these techniques typically give rise to rough noncrystalline edges, which are thought to be the crucial limiting factor to attaining useful performance and on/off current ratios from these devices. In this paper, we demonstrate a technique by which few-layer graphene can be etched along crystallographic directions by down to the underlying substrate - potentially useful for 'carving out' structures in graphene for key applications.

Electrostatic Force Microscopy of Nanofibers and Carbon Nanotubes: Quantitative Analysis Using Theory and Experiment
S. S. Datta, C. Staii, N. J. Pinto, D. R. Strachan, A. T. Johnson, Nanoscale Phenomena in Functional Materials by Scanning Probe Microscopy, Mater. Res. Soc. Symp. Proc. 1025E, 1025-B13-03 (2007).

Electrostatic force microscopy (EFM) is a widely used scanning-probe technique for the characterization of electronic properties of nanoscale samples without the use of electrical contacts. Here we review the basic principles of EFM, developing a quantitative framework by which EFM measurements of extended nanostructures can be understood. In particular, we combine our calculations with experimental data to show that EFM is a direct means of measuring the dielectric properties of carbon nanotubes and conducting or insulating electrospun polyaniline-based nanofibers, as well as thin films of single-stranded DNA, without the use of electrical contacts. Furthermore, we explore a new route towards extending EFM as a means of non-invasively probing the local electronic density of states of carbon nanotubes. This preliminary work could potentially lead to a technique by which the energy band gap of one-dimensional nanostructures could be measured in a straightforward manner.

Real-Time TEM Imaging of the Formation of Crystalline Nanoscale Gaps
D. R. Strachan, D. E. Johnston, B. S. Guiton, S. S. Datta, P. K. Davies, D. A. Bonnell, A. T. Johnson, Physical Review Letters 100, 056805 (2008).

This work was featured in The Virtual Journal of Nanoscale Science and Technology, 18 February 2008.

One of the goals of nanoelectronics is developing single-molecule electronic devices, in which a nanometer-scale molecule is connected to macroscopic leads. This requires the reproducible production of highly ordered nanoscale gaps, in which molecules of interest are electrostatically coupled to nearby gate electrodes. Feedback-controlled electromigration (FCE) of a wire with a nanoscale constriction is emerging as a promising approach to realizing this. To better understand the process by which these nanogaps are produced, we use real-time transmission-electron microscopy to observe crystalline nanogap formation by FCE. We also use finite-element simulations to quantify the improved coupling to a gate electrode of a molecular device incorporating leads formed by this process, compared to devices made using standard nanolithographic techniques.

Functionalized Carbon Nanotubes for Detecting Viral Proteins
Y-B Zhang, M. Kanungo, A. J. Ho, P. Freimuth, D. van der Lelie, M. Chen, S. M. Khamis, S. S. Datta, A. T. Johnson, B. Panessa-Warren, J. A. Misewich, S. S. Wong, Nano Letters 7, 3086 (2007).

This work was featured on Nanotechweb.

Detection of viral proteins using human receptor functionalized carbon nanotubes
M. Chen, S. M. Khamis, S. S. Datta, Y-B Zhang, M. Kanungo, A. J. Ho, P. Freimuth, D. van der Lelie, A. T. Johnson, J. A. Misewich, S. S. Wong, Electroactive and Conductive Polymers and Carbon Nanotubes for Biomedical Applications, Mater. Res. Soc. Symp. Proc. 1065E, 1065-QQ04-05 (2007).

Adenoviruses have been implicated in a wide variety of diseases, including the common cold. Building on our understanding of the structure of these viruses, and by tailoring the surface chemistry of individual single-walled carbon nanotubes (SWNTs), we developed fast, real-time nanoelectronic devices sensitive to the adenovirus protein Ad12 Knob. Using a combination of atomic force microscopy, electronic transport measurements, and biological activity experiments, we show that single-walled carbon nanotubes can be used as biosensors for detecting environmental adenoviruses.

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Other Writings

Structure and Properties of Functional Nanostructures: Carbon Nanotubes, Graphene, and Single-Molecule Transistors
S. S. Datta, Senior Thesis, University of Pennsylvania (2008).

Most of this can be found in the publications listed above - here is the broad overview: low-dimensional nanostructures are emerging as model systems for fundamental studies of quantum transport, as well as promising candidates for novel post-silicon electronic devices incorporating quantum size effects. Key examples of these include few-layer graphene, carbon nanotubes, polymer nanofibers, and tailored single molecules. In this thesis, I present experimental and computational studies of the structure, biochemistry, and electronic properties of these nanomaterials, as well as ways in which they can be engineered for specific applications - specifically, using scanning probe microscopy techniques to study electronic phenomena in few-layer graphene and carbon nanotubes, as well as to elucidate the structure of biochemically-functionalized carbon nanotubes; using computer simulations to investigate key electronic properties of single-molecule transistors; and demonstrating a straightforward chemical technique by which samples of few-layer graphene can be etched along their crystallographic directions, potentially enabling the creation of a variety of new graphene-based nanostructures. Coming soon!

metadatta blog
Once in a while, I blog about results or ideas (mainly scientific) that I think are noteworthy, or about life in academia. I find that blogging is a useful way to keep track of interesting ideas, concepts or events that I come across, and putting this information online means that others can benefit, participate, and collaborate as well.

Penn Triangle, Fall 2005
Established in 1899, the Pennsylvania Triangle is a student-run science and technology publication of the School of Engineering and Applied Science at the University of Pennsylvania. I was the Fall 2005 editor-in-chief of the Triangle, during which I produced this feature issue on nanoscience and nanotechnology.