Overview: Soft Condensed Matter Physics and Nanoscience
I use a variety of experimental tools as well as simulations and simple theoretical models to study problems in the following (click each for details):

Fluid Dynamics
• Multiphase fluid flow through porous media: how does the presence of one fluid or gas trapped in a porous medium affect the flow of another fluid?
• The motion of running reactive droplets: how can fluid droplets that react with the surface they sit on be induced to move?

 Rheology of Complex Fluids
• Rheology of attractive emulsions: how do attractive (versus purely repulsive) interdroplet interactions change the linear and nonlinear rheology of emulsions?
• Rheology of Pickering emulsions: how does coating the droplets in an emulsion with an armor of colloidal particles change its bulk rheology?

Continuum Mechanics of Thin Structures
• Mechanics of Pickering emulsion droplets: is the surface of a fluid droplet coated with an armor of colloidal particles a solid? Can this be understood using continuum mechanics?

Nanoscale Materials
• Crystallographic etching of graphene: can graphene be patterned along crystallographic directions in a straightforward manner?
• Electrostatics of graphene: how do the electrostatic properties of graphene vary as it goes from being mainly surface (few layers) to a bulk material (many layers)?
• Nanoscale biosensors: how can carbon nanotubes be used as biosensors - for example, to detect viruses?

New Probes of Matter
• Scanning probe microscopy: Can new forms of SPM be developed to discover new things about the basic physics of matter?


Peer-Reviewed Publications (Click Titles For Details)
Rheology of Attractive Emulsions
S. S. Datta, D. D. Gerrard, T. S. Rhodes, T. G. Mason, and D. A. Weitz, Physical Review E in press, (2011).
Will be available on the arxiv soon.

Emulsions are suspensions of droplets of one immiscible fluid in another. They are widely used in many technological applications requiring the transport and flow of the dispersed fluid; these include oil recovery, food products, pharmacology, coatings, and cosmetics. In many of these cases, being able to control how the emulsion flows (its rheology) is crucial to optimizing its performance. Surprisingly, very little is known on how exactly the rheology of an emulsion depends on the microscopic interactions between the droplets comprising it. In this paper, we show how changing the interdroplet interactions in an emulsion changes its bulk rheology. Interestingly, changing these interactions can strongly change both the linear and nonlinear viscoelasticity of emulsions.

Buckling and Crumpling of Nanoparticle-Coated Droplets
S. S. Datta, H. C. Shum, and D. A. Weitz, Langmuir 26, 18612 (2010).
Also available at arxiv:1011.4271.

"Pickering" emulsions, suspensions of fluid droplets coated with solid colloidal particles, are of enormous interest because of their remarkable stability and diverse applications. It has been hypothesized that the droplet surfaces are very different from those of regular fluid droplets. In this paper we show that the surfaces of these armored droplets are solid. We do this by coming up with a chemical way to controllably reduce the droplet volumes. This builds up compressive stresses in the droplet surfaces, and we find that a significant fraction of droplets buckle or crumple upon volume reduction, confirming the hypothesis that their interfaces behave like solids. The number of non-spherical droplets as well as the resultant droplet morphology is highly dependent on the amount of volume reduction and the average size of the droplets. All of the morphologies we observe are stable over a period of at least several hours. Many of these are strikingly similar to structures observed or predicted for buckled thin continuum elastic shells. The technique presented in this Letter provides a new and straightforward way to study the deformation behavior of thin fluid-filled granular shells.

Wetting and Energetics in Nanoparticle Etching of Graphene
S. S. Datta, Journal of Applied Physics 108, 024307 (2010).
Also available at arxiv:1008.2203.

This work was featured in The Virtual Journal of Nanoscale Science and Technology, 9 August 2010.

The use of molten metal nanoparticles to etch graphene has recently received quite a bit of attention (see, for example, my previous paper on the subject). However, the microscopic mechanism by which this occurs remains unclear. In this paper, I propose a simple model for this process. In particular, I treat the molten nanoparticle as a fluid droplet that can be forced to move by two factors: a difference between the equilibrium wettability of the graphene and the underlying substrate, or the high surface energy associated with a reactive graphene edge. These basic ingredients give rise to a number of testable predictions for evaluating how significant these factors are in controlling graphene etching. The nice thing is that this model is quite general, and can be applied to other materials systems as well. As an example, I analyze the motion of droplets formed during the evaporation of binary semiconductors, suggesting how a current theory of this process may be extended to better agree with experimental observations.

Gate coupling to nanoscale electronics
S. S. Datta, D. R. Strachan, A. T. Johnson, Physical Review B 79, 205404 (2009).
Also available at arxiv:0812.3177.

This work was featured in The Virtual Journal of Nanoscale Science and Technology, 18 May 2009.

The realization of single-molecule electronic devices, in which a nanometer-scale molecule is connected to macroscopic leads, requires the reproducible production of highly ordered nanoscale gaps in which a molecule of interest is electrostatically coupled to nearby gate electrodes. Understanding how the molecule-gate coupling depends on key parameters is crucial for the development of high-performance devices. Here we directly address this, presenting two- and three-dimensional finite-element electrostatic simulations of the electrode geometries formed using emerging fabrication techniques. We quantify the gate coupling intrinsic to these devices, exploring the roles of parameters believed to be relevant to such devices. These include the thickness and nature of the dielectric used, and the gate screening due to different device geometries. On the single-molecule (~1nm) scale, we find that device geometry plays a greater role in the gate coupling than the dielectric constant or the thickness of the insulator. Compared to the typical uniform nanogap electrode geometry envisioned, we find that non-uniform tapered electrodes yield a significant three orders of magnitude improvement in gate coupling. We also find that in the tapered geometry the polarizability of a molecular channel works to enhance the gate coupling. We anticipate that these results may help guide experimental efforts to realize robust, high-performance single-molecule three-terminal devices at the nanoscale.

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. Cited over 25 times.

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. Cited over 100 times.

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). Cited over 25 times.

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). Cited over 25 times.

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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