Madhu Bhaskaran

Elemental Analogues of Graphene: Silicene, Germanene, Stanene, and Phosphorene

The fascinating electronic and optoelectronic properties of free-standing graphene has led to the exploration of alternative two-dimensional materials that can be easily integrated with current generation of electronic technologies. In contrast to 2D oxide and dichalcogenides, elemental 2D analogues of graphene, which include monolayer silicon (silicene), are fast emerging as promising alternatives, with predictions of high degree of integration with existing technologies. This article reviews this emerging class of 2D elemental materials - silicene, germanene, stanene, and phosphorene--with emphasis on fundamental properties and synthesis techniques. The need for further investigations to establish controlled synthesis techniques and the viability of such elemental 2D materials is highlighted. Future prospects harnessing the ability to manipulate the electronic structure of these materials for nano- and opto-electronic applications are identified.

Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide

We demonstrate that the energy bandgap of layered, high-dielectric α-MoO(3) can be reduced to values viable for the fabrication of 2D electronic devices. This is achieved through embedding Coulomb charges within the high dielectric media, advantageously limiting charge scattering. As a result, devices with α-MoO(3) of ∼11 nm thickness and carrier mobilities larger than 1100 cm(2) V(-1) s(-1) are obtained.

Dielectric resonator nanoantennas at visible frequencies

Drawing inspiration from radio-frequency technologies, we propose a realization of nano-scale optical dielectric resonator antennas (DRAs) functioning in their fundamental mode. These DRAs operate via displacement current in a low-loss high-permittivity dielectric, resulting in reduced energy dissipation in the resonators. The designed nonuniform planar DRA array on a metallic plane imparts a sequence of phase shifts across the wavefront to create beam deflection off the direction of specular reflection. The realized array clearly demonstrates beam deflection at 633 nm. Despite the loss introduced by field interaction with the metal substrate, the proposed low-loss resonator concept is a first step towards nanoantennas with enhanced efficiency. The compact planar structure and technologically relevant materials promise monolithic circuit integration of DRAs.

Elevated temperature anodized Nb2O5: A photoanode material with exceptionally large photoconversion efficiencies

Here, we demonstrate that niobium pentoxide (Nb(2)O(5)) is an ideal candidate for increasing the efficiencies of dye-sensitized solar cells (DSSCs). The key lies in developing a Nb(2)O(5) crisscross nanoporous network, using our unique elevated temperature anodization process. For the same thicknesses of ∼4 μm, the DSSC based on the Nb(2)O(5) layer has a significantly higher efficiency (∼4.1%) when compared to that which incorporates a titanium dioxide nanotubular layer (∼2.7%). This is the highest efficiency among all of the reported photoanodes for such a thickness when utilizing back-side illumination. We ascribe this to a combination of reduced electron scattering, greater surface area, wider band gap, and higher conduction band edge, as well as longer effective electron lifetimes.

Flexible metasurfaces and metamaterials: A review of materials and fabrication processes at micro- and nano-scales

The ability to bend, stretch, and roll metamaterial devices on flexible substrates adds a new dimension to aspects of manipulating electromagnetic waves and promises a new wave of device designs and functionalities. This work reviews terahertz and optical metamaterials realized on flexible and elastomeric substrates, along with techniques and approaches to lend tunability to the devices. Substrate electromagnetic and mechanical characteristics suitable for flexible metamaterials are summarized for readers, followed by fabrication and processing techniques, and finally novel approaches used to-date to attain tunability. Future directions and emerging areas of interests are identified with these promising to transform metamaterial design and translate metamaterials into practical devices.

Characterization of metal contacts for two-dimensional MoS2 nanoflakes

While layered materials are increasingly investigated for their potential in nanoelectronics, their functionality and efficiency depend on charge injection into the materials via metallic contacts. This work explores the characteristics of different metals (aluminium, tungsten, gold, and platinum) deposited on to nanostructured thin films made of two-dimensional (2D) MoS2 flakes. Metals are chosen based on their work functions relative to the electron affinity of MoS2. It is observed, and analytically verified that lower work functions of the contact metals lead to smaller Schottky barrier heights and consequently higher charge carrier injection through the contacts.

Mechanically Tunable Dielectric Resonator Metasurfaces at Visible Frequencies.

Devices that manipulate light represent the future of information processing. Flat optics and structures with subwavelength periodic features (metasurfaces) provide compact and efficient solutions. The key bottleneck is efficiency, and replacing metallic resonators with dielectric resonators has been shown to significantly enhance performance. To extend the functionalities of dielectric metasurfaces to real-world optical applications, the ability to tune their properties becomes important. In this article, we present a mechanically tunable all-dielectric metasurface. This is composed of an array of dielectric resonators embedded in an elastomeric matrix. The optical response of the structure under a uniaxial strain is analyzed by mechanical-electromagnetic co-simulations. It is experimentally demonstrated that the metasurface exhibits remarkable resonance shifts. Analysis using a Lagrangian model reveals that strain modulates the near-field mutual interaction between resonant dielectric elements. The ability to control and alter inter-resonator coupling will position dielectric metasurfaces as functional elements of reconfigurable optical devices.

Field Effect Biosensing Platform Based on 2D α-MoO3

Electrical-based biosensing platforms offer ease of fabrication and simple sensing solutions. Recently, two-dimensional (2D) semiconductors have been proven to be excellent for the fabrication of field effect transistors (FETs) due to their large transconductance, which can be efficiently used for developing sensitive bioplatforms. We present a 2D molybdenum trioxide (MoO3) FET based biosensing platform, using bovine serum albumin as a model protein. The conduction channel is a nanostructured film made of 2D α-MoO3 nanoflakes, with the majority of nanoflake thicknesses being equal to or less than 2.8 nm. The response time is impressively low (less than 10 s), which is due to the high permittivity of the 2D α-MoO3 nanoflakes. The system offers a competitive solution for future biosensing applications.

Experimental demonstration of reflectarray antennas at terahertz frequencies.

Reflectarrays composed of resonant microstrip gold patches on a dielectric substrate are demonstrated for operation at te rahertz frequencies. Based on the relation between the patch size and the reflectio n phase, a progressive phase distribution is implemented on the patch rray to create a reflector able to deflect an incident beam towards a predefine a gle off the specular direction. In order to confirm the validity of th e design, a set of reflectarrays each with periodically distributed 360 ×360 patch elements are fabricated and measured. The experimental results obta ined through terahertz time-domain spectroscopy (THz-TDS) show that up to n early 80% of the incident amplitude is deflected into the desired directi on at an operation frequency close to 1 THz. The radiation patterns of the reflec tarray in TM and TE polarizations are also obtained at different frequen cies. This work presents an attractive concept for developing components a ble to efficiently manipulate terahertz radiation for emerging terahertz com munications. OCIS codes:(300.6495) Spectroscopy, terahertz; (110.5100) Phased-a rray imaging systems; (240.6645) Surface differential reflectance. References and links 1. D. G. Berry, R. G. Malech, and W. A. Kennedy, “The reflectarr ay antenna,” IEEE Trans. Antennas Propag. 11, 645–651 (1963). 2. J. Huang and J. Encinar, Reflectarray Antenna . Wiley-IEEE Press, 2008. 3. J. P. Montgomery, “A microstrip reflectarray antenna elem ent,” Antenna Applications Symposium, University of Illinois (1978). 4. D. M. Pozar and T. A. Metzler, “Analysis of a reflectarray an tenna using microstrip patches of variable size,” Electron. Lett.29,657–658 (1993). 5. D. C. Chang and M. C. Huang, “Multiple-polarization micro strip reflectarray antenna with high efficiency and low cross-polarization,” IEEE Trans. Antennas Propag. 43,829–834 (1995). 6. J. P. Gianvittorio and Y. Rahmat-Samii, “Reconfigurable p atch antennas for steerable reflectarray applications,” IEEE Trans. Antennas Propag. 54,1388–1392 (2006). 7. J. Ginn, B. Lail, J. Alda, and G. Boreman, “Planar infrared binary phase reflectarray,” Opt. Express 33, 779–781 (2008). 8. J. Ginn, B. Lail, and G. Boreman, “Sub-millimeter and infr ared reflectarray,” U. S. Patent 7,623,071 B2 (2009). 9. R. D. Javor, X. D. Wu, and K. Chang, “Design and performance of a microstrip reflectarray antenna,” IEEE Trans. Antennas Propag. 43,932–939 (1995). 10. J. Encinar, M. Arrebola, L. F. de la Fuente, and G. Toso, “A transmit-receive reflectarray antenna for direct broadcast satellite applications,” IEEE Trans. Antennas P ropag.59,3255–3264 (2011). 11. L. Moustafa, R. Gillard, F. Peris, R. Loison, H. Legay, an d E. Girard, “The phoenix cell: a new reflectarray cell with large bandwidth and rebirth capabilities,” IEEE Anten nas Wirel. Propag. Lett. 10,71–74 (2011). 12. J. A. Encinar, “Design of a dual frequency reflectarray us ing microstrip stacked patches of variable size,” Electron. Lett.32,1049–1050 (1996). 13. J. A. Encinar, “Design of two-layer printed reflectarray s using patches of variable size,” IEEE Trans. Antennas Propag.49,1403–1410 (2001). 14. J. A. Encinar, “Recent advances in reflectarray antennas ,” Antennas and Propagation (EuCAP), 2010 Proceedings of the Fourth European Conference on (2010). 15. W. Hu, R. Cahill, J. A. Encinar, R. Dickie, H. Gamble, V. Fu sco, and N. Grant, “Design and measurement of reconfigurable millimeter wave reflectarray cells with nema tic liquid crystal,” IEEE Trans. Antennas Propag. 56, 3112–3117 (2008). 16. S. Ghadarghadr, Z. Hao, and H. Mosallaei, “Plasmonic arr ay nanoantennas on layered substrates: modeling and radiation characteristics,” Opt. Express 17, 18556–18570 (2009) 17. A. Ahmadi, S. Ghadarghadr, and H. Mosallaei, “An optical reflectarray nanoantenna: The concept and design,” Opt. Express18, 123–133 (2010). 18. N. M. Froberg, B. B. Hu, X.-C. Zhang, and D. H. Auston, “Ter ahertz radiation from a photoconducting antenna array,” IEEE J. Quantum Electron. 28, 2291–2301 (1992). 19. M. N. Islam, and M. Koch, “Terahertz patch antenna arrays for indoor communications,” Int. Conference on Next-Generation Wireless Systems 2006 (Dhaka, Bangladesh ) (2006). 20. K. Maki, T. Shibuya, C. Otani, K. Suizu, K. and Kawase, “Te rahertz beam steering via tilted-phase differencefrequency mixing,” Appl. Phys. Express 2, 022301 (2009). 21. Y. Monnai, V. Viereck, H. Hillmer, K. Altmann, C. Jansen, M. Koch, and H. Shinoda, “Terahertz beam steering using structured MEMS surfaces for networked wireless sens ing,” Ninth International Conference on Networked Sensing Systems (INSS) (2012). 22. T. Kleine-Ostmann and T. Nagatsuma, “A review on teraher tz communications research,” J. Infrared Millim. Terahz. Waves32, 143–171 (2011). 23. S. Lucyszyn, “Evaluating surface impedance models for t erahertz frequencies at room temperature,” PIERS Online 3, 554–559 (2007). 24. I. E. Khodasevych, C. M. Shah, S. Sriram, M. Bhaskaran, W. Withayachumnankul, B. S. Y. Ung, H. Lin, W. S. T. Rowe, D. Abbott, and A. Mitchell, “Elastomeric silicone s ubstrates for terahertz fishnet metamaterials,” Appl. Phy. Lett.100, 061101 (2012). 25. S. D. Targonski, and D. M. Pozar, “Analysis and design of a microstrip reflectarray using patches of variable size,” Antennas and Propagation Society International Sym posium, 1994. AP-S. Digest, 1820–1823 (1994). 26. M.-A. Milon, R. Gillard, D. Cadoret, and H. Legay, “Analy sis of mutual coupling for the simulation of reflectarrays radiating cells,” Proc. EuCAP 2006 , Nice, France, 1–6 ( 2006). 27. M.-A. Milon, D. Cadoret, R. Gillard, and H. Legay,“Surro unded-element approach for the simulation of reflectarray radiating cells,” IET Microw. Antennas Propag., 1, 289–293 (2007).Reflectarrays composed of resonant microstrip gold patches on a dielectric substrate are demonstrated for operation at terahertz frequencies. Based on the relation between the patch size and the reflection phase, a progressive phase distribution is implemented on the patch array to create a reflector able to deflect an incident beam towards a predefined angle off the specular direction. In order to confirm the validity of the design, a set of reflectarrays each with periodically distributed 360 × 360 patch elements are fabricated and measured. The experimental results obtained through terahertz time-domain spectroscopy (THz-TDS) show that up to nearly 80% of the incident amplitude is deflected into the desired direction at an operation frequency close to 1 THz. The radiation patterns of the reflectarray in TM and TE polarizations are also obtained at different frequencies. This work presents an attractive concept for developing components able to efficiently manipulate terahertz radiation for emerging terahertz communications.

Plasmon Resonances of Highly Doped Two-Dimensional MoS2

The exhibition of plasmon resonances in two-dimensional (2D) semiconductor compounds is desirable for many applications. Here, by electrochemically intercalating lithium into 2D molybdenum disulfide (MoS2) nanoflakes, plasmon resonances in the visible and near UV wavelength ranges are achieved. These plasmon resonances are controlled by the high doping level of the nanoflakes after the intercalation, producing two distinct resonance peak areas based on the crystal arrangements. The system is also benchmarked for biosensing using bovine serum albumin. This work provides a foundation for developing future 2D MoS2 based biological and optical units.