A. Shchepetov

Interfacial Engineering by Proteins: Exfoliation and Functionalization of Graphene by Hydrophobins

Graphene has attracted vast interest as a new material with many uses. 2] Two-dimensional, crystalline graphene has many advantageous properties, such as extremely high electric and thermal conductivity, high strength, and a large surface area. Many more useful properties can result from graphene assemblies and modification by different functionalities or additional molecules. One of the usual ways to functionalize graphene is chemical modification; however, attempts to modify the surface of graphene in a noncovalent, nondestructive way have also been successful. These methods typically involve the buildup of charge on the graphene surface to enable the stabilization and assembly of the graphene sheets on the basis of electrostatic interactions. In a further step towards more complex functionalities, we have now modified graphene with more specifically interacting coatings consisting of biomolecules. One of the main challenges in the production of graphene is the scalable, controllable, and safe processing and handling of individual graphene sheets. Methods for the fabrication of graphene in a dry environment include the micromechanical cleavage of graphene sheets from graphite and the epitaxial growth of graphene on certain substrates. 11] By these methods, very large entities of single-layer graphene can be produced, but the scalability and handling problems remain. High-yielding solution-based chemical methods that enable the handling of graphene in dispersed form have been proposed; however, they involve the direct oxidation of graphene, which may lower the conductivity of graphene dramatically. Recent reports on the exfoliation of graphene either in pure solvents or in the presence of surfactants offer promise for the production of graphene. The main benefits of solution methods are the better processability and increased safety of graphene when it is dispersed in a liquid instead of being used as a dry powder. The dispersion of graphene into aqueous solutions is especially attractive because of their nonvolatile nature. Herein, we present a method for the exfoliation and functionalization of graphene sheets by an amphiphilic protein. It is known that a microbial adhesion protein, HFBI (Figure 1a), which belongs to a class of proteins called hydrophobins, interacts strongly with hydrophobic surfaces, such as graphite and silicon. The protein has a strongly cross-linked fold containing four disulfide bridges. Its most striking feature is a patch of hydrophobic residues on one face of its structure. Thus, the protein resembles a typical surfactant with a hydrophilic and a hydrophobic part. In solution, hydrophobic interactions between individual proteins lead to the formation of dimers or tetramers. In the vicinity of the interface between water and air, however, assembly of the protein at the interface is strongly preferred, and the protein crystallizes as a 2D lattice. Lateral interactions between surface proteins at interfaces may lead

Reduction of the thermal conductivity in free-standing silicon nano-membranes investigated by non-invasive Raman thermometry

We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality.

Lifetimes of Confined Acoustic Phonons in Ultrathin Silicon Membranes

We study the relaxation of coherent acoustic phonon modes with frequencies up to 500 GHz in ultrathin free-standing silicon membranes. Using an ultrafast pump-probe technique of asynchronous optical sampling, we observe that the decay time of the first-order dilatational mode decreases significantly from ~4.7 ns to 5 ps with decreasing membrane thickness from ~194 to 8 nm. The experimental results are compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wavelength-dependent specularity. Our results provide insight to understand some of the limits of nanomechanical resonators and thermal transport in nanostructures.

Phonons in slow motion: dispersion relations in ultrathin Si membranes.

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as ∼8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than 1 order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano- and microelectromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.

Tuning thermal transport in ultrathin silicon membranes by surface nanoscale engineering

A detailed understanding of the connections of fabrication and processing to structural and thermal properties of low-dimensional nanostructures is essential to design materials and devices for phononics, nanoscale thermal management, and thermoelectric applications. Silicon provides an ideal platform to study the relations between structure and heat transport since its thermal conductivity can be tuned over 2 orders of magnitude by nanostructuring. Combining realistic atomistic modeling and experiments, we unravel the origin of the thermal conductivity reduction in ultrathin suspended silicon membranes, down to a thickness of 4 nm. Heat transport is mostly controlled by surface scattering: rough layers of native oxide at surfaces limit the mean free path of thermal phonons below 100 nm. Removing the oxide layers by chemical processing allows us to tune the thermal conductivity over 1 order of magnitude. Our results guide materials design for future phononic applications, setting the length scale at which nanostructuring affects thermal phonons most effectively.

Ultra-thin free-standing single crystalline silicon membranes with strain control

We report on fabrication and characterization of ultra-thin suspended single crystalline flat silicon membranes with thickness down to 6 nm. We have developed a method to control the strain in the membranes by adding a strain compensating frame on the silicon membrane perimeter to avoid buckling after the release. We show that by changing the properties of the frame the strain of the membrane can be tuned in controlled manner. Consequently, both the mechanical properties and the band structure can be engineered, and the resulting membranes provide a unique laboratory to study low-dimensional electronic, photonic, and phononic phenomena.

Acoustic phonon propagation in ultra-thin Si membranes under biaxial stress field

We report on stress induced changes in the dispersion relations of acoustic phonons propagating in 27nm thick single crystalline Si membranes. The static tensile stress (up to 0.3GPa) acting on the Si membranes was achieved using an additional strain compensating silicon nitride frame. Dispersion relations of thermally activated hypersonic phonons were measured by means of Brillouin light scattering spectroscopy. The theory of Lamb wave propagation is developed for anisotropic materials subjected to an external static stress field. The dispersion relations were calculated using the elastic continuum approximation and taking into account the acousto-elastic effect. We find an excellent agreement between the theoretical and the experimental dispersion relations.

High quality single crystal Ge nano-membranes for opto-electronic integrated circuitry

A thin, flat, and single crystal germanium membrane would be an ideal platform on which to mount sensors or integrate photonic and electronic devices, using standard silicon processing technology. We present a fabrication technique compatible with integrated-circuit wafer scale processing to produce membranes of thickness between 60 nm and 800 nm, with large areas of up to 3.5 mm2. We show how the optical properties change with thickness, including appearance of Fabry-Perot type interference in thin membranes. The membranes have low Q-factors, which allow the platforms to counteract distortion during agitation and movement. Finally, we report on the physical characteristics showing sub-nm roughness and a homogenous strain profile throughout the freestanding layer, making the single crystal Ge membrane an excellent platform for further epitaxial growth or deposition of materials.

Thermoelectric thermal detectors based on ultra-thin heavily doped single-crystal silicon membranes

We present thermal detectors based on 40 nm-thick strain tuned single crystalline silicon membranes shaped into a heater area supported by narrow n- and p-doped beams, which also operate as a thermocouple. The electro-thermal characterization of the devices reveals a noise equivalent power of 13 pW/Hz1/2 and a thermal time constant of 2.5 ms. The high sensitivity of the devices is due to the high Seebeck coefficient of 0.39 mV/K and reduction of thermal conductivity of the Si beams from the bulk value. The performance enables fast and sensitive detection of low levels of thermal power and infrared radiation at room temperature. The devices operate in the Johnson-Nyquist noise limit of the thermocouple, and the performance improvement towards the operation close to the temperature fluctuation limit is discussed.

Thermoelectric bolometers based on silicon membranes

State-of-the-art high performance IR sensing and imaging systems utilize highly expensive photodetector technology, which requires exotic and toxic materials and cooling. Cost-effective alternatives, uncooled bolometer detectors, are widely used in commercial long-wave IR (LWIR) systems. Compared to the cooled detectors they are much slower and have approximately an order of magnitude lower detectivity in the LWIR. We present uncooled bolometer technology which is foreseen to be capable of narrowing the gap between the cooled and uncooled technologies. The proposed technology is based on ultra-thin silicon membranes, the thermal conductivity and electrical properties of which can be controlled by membrane thickness and doping, respectively. The thermal signal is transduced into electric voltage using thermocouple consisting of highly-doped n and p type Si beams. Reducing the thickness of the Si membrane improves the performance (i.e. sensitivity and speed) as thermal conductivity and thermal mass of Si membrane decreases with decreasing thickness. Based on experimental data we estimate the performance of these uncooled thermoelectric bolometers.