Session B16: 2D Devices: Plasmonics and Optoelectronics

GRAPHENE PLASMONICS

Plasmon, the collective free charge carrier oscillation, has been a popular research theme recently mostly associated with surface plasmon in metal nanoparticles. After the discovery of graphene, researchers soon began to study plasmonic effects with or within graphene, for instance, decorating graphene with metal nanoparticles to enhance optical processes via plasmonic field enhancement. Following that, people also gained interests in studying the intrinsic plasmon of graphene. Graphene, a tunable semimetal under field effect, demonstrates tunable plasmon resonances at room temperature, which enables new capabilities beyond those of metal-nanoparticle surface plasmons. In this project, we would like to show intrinsic graphene plasmon resonances in that we experimentally demonstrated polarization dependent and gate-bias tunable plasmon-resonance absorption in the mid-infrared regime of 5-14 um by utilizing an array of graphene nanoribbon resonators. By scaling nanoribbon width and charge densities, we probed graphene plasmons with plasmon resonance energy as high as 0.26 meV (2100 cm$^{\mathrm{-1}})$ for 40 nm wide nanoresonators. The result reveals the intriguing nature of graphene plasmon in graphene nanoribbons where the nanoribbon edge plays critical roles by introducing extra doping and damping the graphene plasmon resonance.   

Optical and Electronic Properties of 2D Graphitic Carbon-Nitride and Carbon Enriched Alloys

The two-dimensional form of graphitic carbon-nitride (gCN) has been successfully synthesized using a simple CVD process. In it's pure form, the carbon to nitrogen ratio is 0.75. By adding a carbon bearing gas to the growth environment, the C/N ratio can be increased, ultimately reaching the pure carbon form: graphene. Unlike attempts at making a 2D alloy system out of BCN, the CN system does not suffer from phase segregation and thus forms a homogeneous alloy. The synthesis approach and electronic and optical properties will be presented for the pure gCN and a selection of alloy compositions.   

Infrared two-wave mixing technique for characterization of graphene THz plasmonic devices

We have studied the heterodyne mixing of two beams from infrared lasers on graphene plasmonic devices and detectors. The nonlinear thermal response of graphene allows us to measure a DC photovoltage that depends on the heterodyne difference frequency and gate voltage. The inversion symmetry of the graphene device is broken by using dissimilar metal contacts to allow a net photo-thermoelectric signal. The power, frequency, and temperature dependence of the photoresponse are used to probe the graphene hot-electron cooling rates and mechanisms. We will discuss the use of photothermal effects in graphene to excite surface plasmons at the difference frequency. The high mobility of the free carriers in graphene is important for this experiment. We have measured exfoliated graphene on SiO2/Si substrate detector and we are working on BN graphene and intercalated SiC graphene devices.   

THz Plasmonics of Quasi-freestanding Bilayer Epitaxial Graphene via H-intercalation

Graphene plasmonics has attracted attention as a suitable platform for tunable THz optoelectronics. THz plasmonic resonances in conventional large-area graphene, however, suffer from low quality factor (Q) because of high carrier scattering rate. This low Q is attributed to charge carrier induced scattering and lower carrier mobility caused by the partially covalent bonding between the silicon carbide (SiC) substrate and the 6$\surd $3 buffer layer between the substrate and EG. Improving the Q of plasmons makes stronger THz resonance effects and also enable THz optoelectronics with fine tunability in frequency via gating. EG on Si-face, semi-insulating 6H-SiC was intercalated in-situ by hydrogen (H$_{\mathrm{2}})$, releasing the buffer layer from SiC forming quasi-freestanding bilayer graphene. H-intercalation time was varied from 0 -- 75 minutes and structural, electrical and optical properties were explored, revealing at long H-intercalation durations high carrier mobility (3000-4000 cm$^{\mathrm{2}}$/Vs) and high sheet carrier concentration (1E13 cm$^{\mathrm{-2}})$ independent of carrier mobility. Far IR simultaneous transmission/reflection measurements revealed a narrow frequency response with line widths ($\gamma )$ smaller in H-intercalated EG (30cm$^{\mathrm{-1}})$ than observed in pristine EG (\textgreater 100cm$^{\mathrm{-1}})$ consistent with the improved mobility.   

Graphene Josephson Junction Single Photon Detector

Single photon detectors (SPDs) have found use across a wide array of applications depending on the wavelength to which they are sensitive. Graphene, because of its linear, gapless dispersion near the Dirac point, has a flat, wide bandwidth absorption that can be enhanced to near 100$\%$ through the use of resonant structures making it a promising candidate for broadband SPDs. Upon absorbing a photon in the optical to mid-infrared range, a small ($\sim$10 $\mu$m$^2$) sheet of graphene at cryogenic temperatures can experience a significant increase in electronic temperature due to its extremely low heat capacity. At 1550 nm, for example, calculations show that the temperature could rise by as much as 500$\%$. This temperature increase could be detected with near perfect quantum efficiency by making the graphene the weak link in a Josephson junction (JJ). We present a theoretical model demonstrating that such a graphene JJ SPD could operate at the readily achievable temperature of 3 K with near zero dark count, sub-50 ps timing jitter, and sub-5 ns dead time and report on the progress toward experimentally realizing the device.   

Imaging height fluctuations in free-standing graphene membranes

We present a technique based on multi-wavelength interference microscopy to measure the heights of observed ripples in free-standing graphene membranes. Graphene membranes released from a transparent substrate produce interference fringes when viewed in the reflection mode of an inverted microscope(Blees et. al. Nature 524 (7564): 204-207 (2015)). The fringes correspond to corrugation of the membrane as it floats near an interface. A single set of fringes is insufficient to uniquely determine the height profile, as a given fringe spacing can correspond to an increase or decrease in height by $\lambda /2$. Imaging at multiple wavelengths resolves the ambiguities in phase, and enables unique determination of the height profile of the membrane (Schilling et. al.Phys. Rev. E, 69:021901, 2004). We utilize this technique to map out the height fluctuations in free-standing graphene membranes to answer questions about fundamental mechanical properties of two-dimensional materials.   

Metal Ion Intercalated graphitic as Transparent Electrodes

To best utilize the performance of graphene based transparent electrodes, we novelized Li-ion intercalation in graphene, and achieved highest performance of carbon based transparent electrodes. Transmission as high as 91.7{\%} with a sheet resistance of 3.0 ohm/sq is achieved for 19-layer LiC6, significantly higher than any other continuous transparent electrodes. The unconventional modification of ultrathin graphite optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. To achieve low cost, large scale graphene-based transparent electrodes, we further developed Na-ion intercalated printed reduced graphene oxide (RGO) film. The larger layer-layer distance of RGO allows Na-ion intercalation, leading to simultaneously much higher DC conductivity and higher optical transmittance. Typical increase of transmittance from 36{\%} to 79{\%} and decrease of sheet resistance from 83 kohms/sq to 311 ohms/sq in the printed network was observed. This study demonstrated the great potential of metal-ion intercalation to improve the performance of graphene-based materials for transparent conductor applications.   

Tunable Broadband Printed Carbon Transparent Conductor

Transparent conductors have been widely applied in solar cells, transparent smart skins, and sensing/imaging antennas, etc. Carbon-based transparent conductor has attracted great attention for its low cost and broad range transparency. Ion intercalation has been known to highly dope graphitic materials, thereby tuning materials' optoelectronic properties. For the first time, we successfully tune the optical transmittance of a reduced graphene oxide (RGO)/CNT network from mid-IR range to visible range by means of Li-ion intercalation/deintercalation. We also observed a simultaneous increase of the electrical conductivity with the Li-ion intercalation. This printed carbon hybrid thin film was prepared through all solution processes and was easily scalable. This study demonstrates the possibility of using ion intercalation for low cost, tunable broadband transparent conductors.   

Design Two-dimensional Materials with Superb Electronic and Optoelectronic Properties: The case of SiS

Two-dimensional (2D) semiconductors have many unique electronic and optoelectronic properties that is suitable for novel device applications. Most of the current study are focused on group IV or transition metal chalcogenides. In this study, using atomic transmutation and global optimization methods, we identified two group IV-VI 2D materials, Pma2-SiS and silicene sulfide that can overcome shortcomings encountered in conventional 2D semiconducttord. Pma2-SiS is found to be both chemically, energetically, and thermally stable. Most importantly, Pma2-SiS has unique electronic and optoelectronic properties, including direct bandgaps suitable for solar cells, good mobility for nanoelectronics, good flexibility of property tuning by layer thickness and strain appliance, and good air stability as well. Therefore, Pma2-SiS is expected to be a very promising 2D material in the field of 2D electronics and optoelectronics. Silicene sulfide also shows similar properties. We believe that the designing principles and approaches used to identify these materials have great potential to accelerate future finding of new functional materials within the 2D families.   

Tunable ambipolar polarization-sensitive photodetectors based on high anisotropy ReSe$_{2}$

Atomically-thin 2D layered transition metal dichalcogenides (TMDs) have been extensively studied recently because of their intriguing physical properties and promising applications in nanoelectronic devices. Among them, ReSe$_{2\, }$is a material that exhibits a stable distorted 1T phase and strong in-plane anisotropy. Here, the anisotropic nature of ReSe$_{2}$ is revealed by Raman scattering under linearly polarized excitations. Utilizing high-quality ReSe$_{2}$ nanosheets, we are able to build top-gate ReSe$_{2}$ field-effect transistors which show an excellent on/off current ratio exceeding 10$^{7}$ and a well-developed current saturation at room temperature. Importantly, the successful synthesis of ReSe$_{2}$ directly onto hexagonal boron nitride substrates has effectively improved the electron motility over 100 times and the hole mobility over 50 times at low temperatures. Remarkably, the ReSe$_{2\, }$based photodetectors show a polarization-sensitive photo-responsivity due to the intrinsic linear dichroism originated from high in-plane optical anisotropy. With a back gate the linear dichroism photodetection can be unambiguously tuned both in the electron and hole regime. The appealing physical properties of ReSe$_{2}$ demonstrated in this study identify it as an emerging candidate for electronic and optoelectronic applications.   

Silicon-nitride photonic circuits interfaced with monolayer MoS2

Monolayers of transition metal dichalcogenides exhibit interesting low-dimensional opto-electronic phenomena and large optical interactions. Harnessing these features for modulating light requires interfacing these monolayer semiconductors with photonic devices. Here, we show the integration of monolayer molybdenum disulphide (MoS$_2$) with silicon nitride ring microresonators using a visco-elastic layer transfer~\footnote{G. Wei, T. K. Stanev, D. A. Czaplewski, I. W. Jung, and N. P. Stern. \textit{Appl. Phys. Lett.} \textbf{107}, 091112 (2015)}. Cavity transmission is used to measure the coupling of the monolayer evanescently coupled to the ring resonator. A linear absorption coefficient of 850 dB/cm is observed in this geometry, which is larger than that of graphene and black phosphorus with the same thickness. These assembly methods can be applied to a diverse catalog of monolayer materials for assembling hybrid optoelectronic devices over a wide spectral range.   

Plasmonic Hot Electron Induced Photocurrent Response at MoS$_{\mathrm{2}}$-Metal Junctions

We investigate the photocurrent generation mechanisms at few-layer MoS$_{\mathrm{2}}$-metal junctions through wavelength- and polarization-dependent scanning photocurrent measurements. When laser energy is above the direct bandgap of MoS$_{\mathrm{2}}$, the maximum photocurrent response is observed when incident laser polarization direction is parallel to the metal electrode due to photovoltaic effect. On the contrary, when illuminated by laser with energy below the direct bandgap of MoS$_{\mathrm{2}}$, the strongest photocurrent response occurs when incident laser is polarized perpendicular to the metal electrode. Further studies demonstrate that light absorption by the plasmonic metal electrode is polarization-dependent, which creates hot electron-hole pairs and subsequently inject into MoS$_{\mathrm{2}}$. These studies shed light on future design rules of two-dimensional material based optoelectronic devices through surface plasmon resonances.   

Spin-Polarized Transport on Photo-Asisted Bilayer Graphene Ribbons

We show how both transmission and spin polarization [1,2] behave in bilayer graphene ribbons in contact with a ferromagnetic insulator while a laser is applied to the ribbon. Using a $\pi$-orbital tight-binding model as a low energy approximation [1] and the Tien-Gordon [3] formalism we explore how these systems behave when the ribbon is photo-assisted with a laser. For particular values of the laser parameters, the Fano antiresonance are removed enhancing the transmission while for others spin-polarized transport will arise. \begin{thebibliography}{0} \bibitem{ORCP} P. A. Orellana, L. Rosales, L. Chico, and M. Pacheco, J. Appl. Phys. \textbf{113}, 213710 (2013). \bibitem{SOB} J. F. Song, Y. Ochiai, and J. P. Bird, Appl. Phys. Lett. \textbf{82}, 4561 (2003). \end{thebibliography}   

Graphene/MoS$_{\mathrm{2}}$ heterostructures for optoelectronics applications

Graphene and other atomically thin materials can be combined to make novel ultra-thin devices that are suitable for flexible substrates. However, fabricating these heterostructures is a challenge. Most previous work was done by stacking monolayers exfoliated from bulk materials$^{\mathrm{\thinspace }}$[1], which is a very time-consuming, low-yield method. Large-area monolayer can also be grown by CVD and stacked, as demonstrated by the successful transfer of graphene on as-grown MoS$_{\mathrm{2\thinspace }}$[2], yet the optical properties of some materials like MoS$_{\mathrm{2}}$ may be degraded by the processing required to detach them from the growth substrate, thereby limiting options in device architecture. Here we develop a method to transfer, align and stack large flakes and films of MoS2 and graphene after transferring both from the growth substrate onto an arbitrary substrate. The Raman and photoluminescence measurements show that the optical properties of the stacked monolayers are not degraded, making this method viable for fabrication of optoelectronics devices. .[1] A.K. Geim, et al., Nature, 499 (2013) 419. [2] L.L. Yu, et al., Nano Letters, 14 (2014) 3055.   

Controlled growth, growth mechanism, and device applications of two-dimensional WSe2

Atomically thin 2D transition metal dichalcogenides have attracted lots of attention recently. Here we will present our progress on the controlled growth of 2D WSe2. Vapor phase methods for the growth of large single crystalline WSe2 with lateral sizes up to tens of micrometers will be discussed. Substrate atomic-step-guided nucleation and growth of aligned WSe2 on single crystalline sapphire substrate will also be presented. In addition, by reducing the supply of source materials, we observed a novel screw-dislocation-driven growth of 2D few layer and pyramid-like WSe2 flakes. Then, we will discuss device applications of CVD WSe2. We show that the device characteristics of CVD WSe2 can be tuned into either p-type or ambipolar behavior, by changing the types of contact metals. We further developed an efficient method to convert as-grown semiconducting 2H-phase WSe2 into metallic 1T-phase WSe2, by controlled reacting with n-butyl lithium (n-BuLi). By using metallic WSe2 as contact regimes and intact semiconducting WSe2 as channel regimes, we successfully made ohmic contacted WSe2 transistors and achieved a hole mobility of 66 cm2/V.s and on/off ratio of 10\textasciicircum 7 for monolayer CVD WSe2.