arXiv subjects

Damon B. Farmer

Publications and source records attributed to Damon B. Farmer.

At least 19 recordsLinked to original sources

arXiv2018

Intrinsically ultrastrong plasmon-exciton interactions in crystallized films of carbon nanotubes

In cavity quantum electrodynamics, optical emitters that are strongly coupled to cavities give rise to polaritons with characteristics of both the emitters and the cavity excitations. We show that carbon nanotubes can be crystallized into chip-scale, two-dimensionally ordered films and that this new material enables intrinsically ultrastrong emitter-cavity interactions: rather than interacting with external cavities, nanotube excitons couple to the near-infrared plasmon resonances of the nanotubes themselves. Our polycrystalline nanotube films have a hexagonal crystal structure, ~25 nm domains, and a 1.74 nm lattice constant. With this extremely high nanotube density and nearly ideal plasmon-exciton spatial overlap, plasmon-exciton coupling strengths reach 0.5 eV, which is 75% of the bare exciton energy and a near record for room-temperature ultrastrong coupling. Crystallized nanotube films represent a milestone in nanomaterials assembly and provide a compelling foundation for high-ampacity conductors, low-power optical switches, and tunable optical antennas.

cond-mat.mes-hall↗

arXiv2018

Graphene-enabled, directed nanomaterial placement from solution for large-scale device integration

Controlled placement of nanomaterials at predefined locations with nanoscale precision remains among the most challenging problems that inhibit their large-scale integration in the field of semiconductor process technology. Methods based on surface functionalization have a drawback where undesired chemical modifications can occur and deteriorate the deposited material. The application of electric-field assisted placement techniques eliminates the element of chemical treatment; however, it requires an incorporation of conductive placement electrodes that limit the performance, scaling, and density of integrated electronic devices. Here, we report a method for electric-field assisted placement of solution-processed nanomaterials by using large-scale graphene layers featuring nanoscale deposition sites. The structured graphene layers are prepared via either transfer or synthesis on standard substrates, then are removed without residue once nanomaterial deposition is completed, yielding material assemblies with nanoscale resolution that cover surface areas larger than 1mm2. In order to demonstrate the broad applicability, we have assembled representative zero-, one-, and two-dimensional semiconductors at predefined substrate locations and integrated them into nanoelectronic devices. This graphene-based placement technique affords nanoscale resolution at wafer scale, and could enable mass manufacturing of nanoelectronics and optoelectronics involving a wide range of nanomaterials prepared via solution-based approaches.

physics.app-ph↗

arXiv2017

Strong and broadly tunable plasmon resonances in thick films of aligned carbon nanotubes

Low-dimensional plasmonic materials can function as high quality terahertz and infrared antennas at deep subwavelength scales. Despite these antennas' strong coupling to electromagnetic fields, there is a pressing need to further strengthen their absorption. We address this problem by fabricating thick films of aligned, uniformly sized carbon nanotubes and showing that their plasmon resonances are strong, narrow, and broadly tunable. With thicknesses ranging from 25 to 250 nm, our films exhibit peak attenuation reaching 70%, quality factors reaching 9, and electrostatically tunable peak frequencies by a factor of 2.3x. Excellent nanotube alignment leads to the attenuation being 99% linearly polarized along the nanotube axis. Increasing the film thickness blueshifts the plasmon resonators down to peak wavelengths as low as 1.4 micrometers, promoting them to a new near-infrared regime in which they can both overlap the S11 nanotube exciton energy and access the technologically important infrared telecom band.

physics.optics↗

arXiv2017

Coherent plasmon and phonon-plasmon resonances in carbon nanotubes

Carbon nanotubes provide a rare access point into the plasmon physics of one-dimensional electronic systems. By assembling purified nanotubes into uniformly sized arrays, we show that they support coherent plasmon resonances, that these plasmons enhance and hybridize with phonons, and that the phonon-plasmon resonances have quality factors as high as 10. Because coherent nanotube plasmonics can strengthen light-matter interactions, it provides a compelling platform for surface-enhanced infrared spectroscopy and tunable, high-performance optical devices at the nanometer scale.

physics.optics↗

arXiv2016

Coupling-Enhanced Broadband Mid-Infrared Light Absorption in Graphene Plasmonic Nanostructures

Plasmons in graphene nanostructures show great promise for mid-infrared applications ranging from a few to tens of microns. However, mid-infrared plasmonic resonances in graphene nanostructures are usually weak and narrow-banded, limiting their potential in light manipulation and detection. Here we investigate the coupling among graphene plasmonic nanostructures and further show that by engineering the coupling, enhancement of light-graphene interaction strength and broadening of spectral width can be achieved simultaneously. Leveraging the concept of coupling, we demonstrate a hybrid 2-layer graphene nanoribbon array which shows 5 to 7% extinction within the entire 8 to 14 μm (~700 to 1250 cm-1) wavelength range, covering one of the important atmosphere "infrared transmission windows". Such coupled hybrid graphene plasmonic nanostructures may find applications in infrared sensing and free-space communications.

cond-mat.mes-hall↗

arXiv2015

Plasmon coupling in extended structures: Graphene superlattice nanoribbon arrays

Interactions between localized plasmons in proximal nanostructures is a well-studied phenomenon. Here we explore plasmon plasmon interactions in connected extended systems. Such systems can now be easily produced using graphene. Specifically we employ the finite element method to study such interactions in graphene nanoribbon arrays with a periodically modulated electrochemical potential or number of layers. We find a rich variation in the resulting plasmonic resonances depending on the dimensions and the electrochemical potentials (doping) of the nanoribbon segments and the involvement of transverse and longitudinal plasmon interactions. Unlike predictions based of the well-known "orbital hybridization model", the energies of the resulting hybrid plasmonic resonances of the extended system can lie between the energies of the plasmons of the individual components. The results demonstrate the wide range tunability of the graphene plasmons and can help to design structures with desired spectra, which can be used to enhance optical fields in the infrared region of the electromagnetic spectrum.

cond-mat.mes-hall↗

arXiv2014

Electronic transport and device prospects of monolayer molybdenum disulphide grown by chemical vapour deposition

Layered transition metal dichalcogenides display a wide range of attractive physical and chemical properties and are potentially important for various device applications. Here we report the electronic transport and device properties of monolayer molybdenum disulphide (MoS2) grown by chemical vapour deposition (CVD). We show that these devices have the potential to suppress short channel effects and have high critical breakdown electric field. However, our study reveals that the electronic properties of these devices are at present, severely limited by the presence of a significant amount of band tail trapping states. Through capacitance and ac conductance measurements, we systematically quantify the density-of-states and response time of these states. Due to the large amount of trapped charges, the measured effective mobility also leads to a large underestimation of the true band mobility and the potential of the material. Continual engineering efforts on improving the sample quality are needed for its potential applications.

cond-mat.mes-hall↗

arXiv2012

High-frequency performance of scaled carbon nanotube array field-effect transistors

We report the radio-frequency performance of carbon nanotube array transistors that have been realized through the aligned assembly of highly separated, semiconducting carbon nanotubes on a fully scalable device platform. At a gate length of 100 nm, we observe output current saturation and obtain as-measured, extrinsic current gain and power gain cut-off frequencies, respectively, of 7 GHz and 15 GHz. While the extrinsic current gain is comparable to the state-of-the-art the extrinsic power gain is improved. The de-embedded, intrinsic current gain and power gain cut-off frequencies of 153 GHz and 30 GHz are the highest values experimentally achieved to date. We analyze the consistency of DC and AC performance parameters and discuss the requirements for future applications of carbon nanotube array transistors in high-frequency electronics.

cond-mat.mtrl-sci↗

arXiv2011

Enhanced Performance in Epitaxial Graphene FETs with Optimized Channel Morphology

This letter reports the impact of surface morphology on the carrier transport and RF performance of graphene FETs formed on epitaxial graphene films synthesized on SiC substrates. Such graphene exhibits long terrace structures with widths between 3-5 μm and steps of 10\pm2 nm in height. While a carrier mobility above 3000 cm2/Vs at a carrier density of 1e12 cm-2 is obtained in a single graphene terrace domain at room temperature, the step edges can result in a vicinal step resistance of ~21 kΩ.μm. By orienting the transistor layout so that the entire channel lies within a single graphene terrace, and reducing the access resistance associated with the ungated part of the channel, a cut-off frequency above 200 GHz is achieved for graphene FETs with channel lengths of 210 nm, which is the highest value reported on epitaxial graphene thus far.

cond-mat.mes-hall↗

arXiv2010

Multi-carrier Transport in Epitaxial Multi-layer Graphene

Variable-field Hall measurements were performed on epitaxial graphene grown on Si-face and C-face SiC. The carrier transport involves essentially a single-type of carrier in few-layer graphene, regardless of SiC face. However, in multi-layer graphene (MLG) grown on C-face SiC, the Hall measurements indicated the existence of several groups of carriers with distinct mobilities. Electrical transport in MLG can be properly described by invoking three independent conduction channels in parallel. Two of these are n- and p-type, while the third involves nearly intrinsic graphene. The carriers in this lightly doped channel have significantly higher mobilities than the other two.

cond-mat.mes-hall↗

arXiv2010

Wafer-scale Epitaxial Graphene Growth on the Si-face of Hexagonal SiC (0001) for High Frequency Transistors

Up to two layers of epitaxial graphene have been grown on the Si-face of two-inch SiC wafers exhibiting room-temperature Hall mobilities up to 1800 cm^2/Vs, measured from ungated, large, 160 micron x 200 micron Hall bars, and up to 4000 cm^2/Vs, from top-gated, small, 1 micron x 1.5 micron Hall bars. The growth process involved a combination of a cleaning step of the SiC in a Si-containing gas, followed by an annealing step in Argon for epitaxial graphene formation. The structure and morphology of this graphene has been characterized using AFM, HRTEM, and Raman spectroscopy. Furthermore, top-gated radio frequency field effect transistors (RF-FETs) with a peak cutoff frequency fT of 100 GHz for a gate length of 240 nm were fabricated using epitaxial graphene grown on the Si face of SiC that exhibited Hall mobilities up to 1450 cm^2/Vs from ungated Hall bars and 1575 cm^2/Vs from top-gated ones. This is by far the highest cut-off frequency measured from any kind of graphene.

cond-mat.mtrl-sci↗

arXiv2010

Efficient narrow-band light emission from a single carbon nanotube p-n diode

Electrically-driven light emission from carbon nanotubes could be exploited in nano-scale lasers and single-photon sources, and has therefore been the focus of much research. However, to date, high electric fields and currents have been either required for electroluminescence, or have been an undesired side effect, leading to high power requirements and low efficiencies. In addition, electroluminescent linewidths have been broad enough to obscure the contributions of individual optical transitions. Here, we report electrically-induced light emission from individual carbon nanotube p-n diodes. A new level of control over electrical carrier injection is achieved, reducing power dissipation by a factor of up to 1000, and resulting in zero threshold current, negligible self-heating, and high carrier-to- photon conversion efficiencies. Moreover, the electroluminescent spectra are significantly narrower (ca. 35 meV) than in previous studies, allowing the identification of emission from free and localized excitons.

cond-mat.mes-hall↗

arXiv2010

100 GHz Transistors from Wafer Scale Epitaxial Graphene

High-performance graphene field-effect transistors have been fabricated on epitaxial graphene synthesized on a two-inch SiC wafer, achieving a cutoff frequency of 100 GHz for a gate length of 240 nm. The high-frequency performance of these epitaxial graphene transistors not only shows the highest speed for any graphene devices up to date, but it also exceeds that of Si MOSFETs at the same gate length. The result confirms the high potential of graphene for advanced electronics applications, marking an important milestone for carbon electronics.

cond-mat.mtrl-sci↗

arXiv2010

Graphene field-effect-transistors with high on/off current ratio and large transport band gap at room temperature

Graphene is considered to be a promising candidate for future nano-electronics due to its exceptional electronic properties. Unfortunately, the graphene field-effect-transistors (FETs) cannot be turned off effectively due to the absence of a bandgap, leading to an on/off current ratio typically around 5 in top-gated graphene FETs. On the other hand, theoretical investigations and optical measurements suggest that a bandgap up to a few hundred meV can be created by the perpendicular E-field in bi-layer graphenes. Although previous carrier transport measurements in bi-layer graphene transistors did indicate a gate-induced insulating state at temperature below 1 Kelvin, the electrical (or transport) bandgap was estimated to be a few meV, and the room temperature on/off current ratio in bi-layer graphene FETs remains similar to those in single-layer graphene FETs. Here, for the first time, we report an on/off current ratio of around 100 and 2000 at room temperature and 20 K, respectively in our dual-gate bi-layer graphene FETs. We also measured an electrical bandgap of >130 and 80 meV at average electric displacements of 2.2 and 1.3 V/nm, respectively. This demonstration reveals the great potential of bi-layer graphene in applications such as digital electronics, pseudospintronics, terahertz technology, and infrared nanophotonics.

cond-mat.mtrl-sci↗

arXiv2009

Dual Gate Graphene FETs with fT of 50 GHz

A dual-gate graphene field-effect transistors is presented, which shows improved RF performance by reducing the access resistance using electrostatic doping. With a carrier mobility of 2700 cm2/Vs, a cutoff frequency of 50 GHz is demonstrated in a 350-nm gate length device. This fT value is the highest frequency reported to date for any graphene transistor, and it also exceeds that of Si MOSFETs at the same gate length, illustrating the potential of graphene for RF applications.

cond-mat.mes-hall↗

arXiv2009

Utilization of a Buffered Dielectric to Achieve High Field-Effect Carrier Mobility in Graphene Transistors

We utilize an organic polymer buffer layer between graphene and conventional gate dielectrics in top-gated graphene transistors. Unlike other insulators, this dielectric stack does not significantly degrade carrier mobility, allowing for high field-effect mobilities to be retained in top-gate operation. This is demonstrated in both two-point and four-point analysis, and in the high-frequency operation of a graphene transistor. Temperature dependence of the carrier mobility suggests that phonons are the dominant scatterers in these devices.

cond-mat.mtrl-sci↗

arXiv2008

Chemical Doping and Electron-Hole Conduction Asymmetry in Graphene Devices

We investigate polyethylene imine and diazonium salts as stable, complementary dopants on graphene. Transport in graphene devices doped with these molecules exhibits asymmetry in electron and hole conductance. The conductance of one carrier is preserved, while the conductance of the other carrier decreases. Simulations based on nonequilibrium Green's function formalism suggest that the origin of this asymmetry is imbalanced carrier injection from the graphene electrodes caused by misalignment of the electrode and channel neutrality points.

cond-mat.mes-hall↗

arXiv2008

Operation of Graphene Transistors at GHz Frequencies

Top-gated graphene transistors operating at high frequencies (GHz) have been fabricated and their characteristics analyzed. The measured intrinsic current gain shows an ideal 1/f frequency dependence, indicating an FET-like behavior for graphene transistors. The cutoff frequency fT is found to be proportional to the dc transconductance gm of the device. The peak fT increases with a reduced gate length, and fT as high as 26 GHz is measured for a graphene transistor with a gate length of 150 nm. The work represents a significant step towards the realization of graphene-based electronics for high-frequency applications.

cond-mat.other↗