Dr Lu's research focuses on nanotechnologies, including micro/nano-electro-mechanical sensors and actuators, nano-scale energy conversion devices, biomedical devices, novel nano materials, etc. He is looking for highly motivated Ph.D. graduate students who are majoring in applied physics, electrical engineering, mechanical engineering, chemistry, materials science engineering, biomedical engineering, or other closely related areas to join his research team. They are always looking for dedicated undergraduate or master students to join them, to do creative work in rapidly growing field and generate co-authored publications.

1. MEMS/NEMS based novel biomedical devices

The ability to detect bio-molecule at ultra-low concentrations (e.g. atto-molar) will enable the possibility of detecting diseases earlier than ever before. A critical challenge for any new bio-sensing technology is to optimize two metrics --- shorter analysis time, and higher concentration sensitivity in clinically relevant small volumes. Moreover, practical considerations are equally important: simplicity of use, mass producible (low cost), and ease of integration within the clinical structure. Compared with other methods, nano-electro-mechanical system (NEMS) based bio-sensors are promising in clinical diagnostics because of their extremely high mass sensitivity, fast response time and the capability of integration on chip. They have demonstrated a low concentration DNA (atto-molar sensitivity) optically interrogated ultrasonic mechanical mass sensor, which has ordered nanowire (NW) array on top of a bilayer membrane. This method represents a mass-based platform technology that can sense molecules at low concentrations, which could be useful for early-stage disease detection. They can develop this sensor further to measure an array of biomarkers (e.g. DNA or proteins), by providing both the needed specificity and sensitivity in physiological disease (e.g. cancer) detection.

Recommended references:

• B. Ilic et al., Enumeration of DNA molecules bound to a nanomechanical oscillator. Nano Letters 5, 925-929 (2005).
• T. P. Burg et al., Weighing of biomolecules, single cells and single nanoparticles in fluid. Nature 446, 1066-1069 (2007).
• H. G. Craighead, Nanoelectromechanical systems. Science 290, 1532-1535 (2000).
• A. K. Naik, M. S. Hanay, W. K. Hiebert, X. L. Feng, M. L. Roukes, Towards single-molecule nanomechanical mass spectrometry. Nature Nanotechnology 4, 445-450 (2009).
• Y. Lu, S. Peng, D. Luo, A. Lal, Low-concentration mechanical biosensor based on a photonic crystal nanowire array. Nature Communications 2, 578 (2011).

2. Nonlinearity engineering for nano-electro-mechanical system

A nano-electro-mechanical system (NEMS) that integrates various nano-scale mechanical sensors onto a single chip, has attracted exceptional interest among research and engineering communities. Among other remarkable characteristics, these tiny mechanical structures possess strong nonlinear properties. Typically, device nonlinearity is intentionally avoided, because higher amplitude fluctuations in the nonlinear region could be translated into frequency variability. Recently, they found that nonlinearity bifurcation could be used to stabilize our membrane oscillator with nanowire (NW) arrays. As a consequence, they propose to use their NEMS device as a platform to investigate its nonlinear dynamics and gain a deeper understanding of the origins of that nonlinearity. This could present a general mechanism to assist oscillation frequency stabilization and improve NEMS resonator performance for bio-molecular sensing applications. These nanowire arrays could also be embedded into suspended micro-channels. Coupled with the nonlinearity-assisted frequency stabilization result, the sensitivity of the mass sensor could be improved by orders of magnitude.

Recommended references:

• A. K. Naik, M. S. Hanay, W. K. Hiebert, X. L. Feng, M. L. Roukes, Towards single-molecule nanomechanical mass spectrometry. Nature Nanotechnology 4, 445-450 (2009).
• I. Kozinsky, H. W. C. Postma, O. Kogan, A. Husain, M. L. Roukes, Basins of attraction of a nonlinear nanomechanical resonator. Physical Review Letters 99, 207201 (2007).
• D. Antonio, D. H. Zanette, D. Lopez, Frequency stabilization in nonlinear micromechanical oscillators. Nature Communications 3, 806 (2012).
• Y. Lu, S. Peng, D. Luo, A. Lal, Low-concentration mechanical biosensor based on a photonic crystal nanowire array. Nature communications 2, 578 (2011).
• Y. Lu, and A. Lal, Nonlinearity-assisted frequency stabilization for nanowire array membrane oscillator.Proceedings of the 26th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2013), pp. 653-656, Taipei, Taiwan, January 20-25, 2013.

3. Nano-electro-mechanical system based on novel two dimensional nano-materials

Two-dimensional (2D) nano-materials, such as molybdenum disulfide (MoS₂) and graphene, have atomic or molecular thickness, exhibiting promising applications in nano-electro-mechanical systems. Graphene is a one-atom thick carbon sheet, with atoms arranged in a regular hexagonal pattern. Molybdenum disulfide (MoS₂) belongs to transition metal dichalcogenides (TMD) semiconductor family YX₂ (Y=Mo, W; X=S, Se, Te), with a layered structure. These 2D nano-materials can be integrated into nano-electro-mechanical systems, enabling ultra-sensitive mechanical mass sensors, with single molecule or even single atom sensitivities. Moreover, the mechanical resonators based on these 2D nano-materials would be a perfect platform to investigate quantum mechanics, opto-mechanics, material internal friction force, nonlinear physics, etc.

Recommended references:

• J. S. Bunch et al., Electromechanical resonators from graphene sheets. Science 315, 490-493 (2007).
• R. A. Barton et al., Photothermal self-oscillation and laser cooling of graphene optomechanical systems. Nano Letters 12, 4681-4686 (2012).
• Radisavljevicb, Radenovica, Brivioj, Giacomettiv, Kisa, Single-layer MoS₂ transistors. Nature Nanotechnology 6, 147-150 (2011).
• M. Osada, T. Sasaki, Two-dimensional dielectric nanosheets: Novel nanoelectronics from nanocrystal building blocks. Advanced Materials 24, 210-228 (2012).

4. High-throughput Nano-manufacturing

Nanolithography is important for nano-scale device fabrication, both in academia and in the semiconductor industry. The fundamental challenge in developing nanolithography systems is to create a system with high resolution (sub-20 nm), high throughput (4-inch wafer exposure within 30 seconds), high controllability and low cost. Dr Lu has demonstrated the proof of concept for a new lithography system --- Radioisotope-powered Parallel Electron Lithography (RIPEL). This system uses high energy electrons emitted from radioisotope thin films to expose resist through a stencil mask. The RIPEL system simultaneously exposes the entire substrate, making it a parallel system, as opposed to the serial raster scanning electron beam lithography system. This will enable the rapid exposure of nano-scale features over very large substrates. However, there are still several significant hurdles in preventing RIPEL’s practical uses. We will study these hurdles and propose developing building blocks to overcome these hurdles, through radioisotope electron sources development, new mask design, and investigation of new electron resist. These building blocks will enhance RIPEL’s throughput by four orders of magnitude. The progress will significantly benefit the areas that need volumetric nanostructures with high spatial density and good controllability, such as plasmonics, nanoelectronics, micro/nano-electro-mechanical systems, nano-structured thin film solar cells, biomedical devices, etc.

Recommended references:

• Y. Lu, N. Yoshimizu, A. Lal, Self-powered near field electron lithography. Journal of Vacuum Science & Technology B 27, 2537-2541 (2009).
• Y. Lu, A. Lal, Vacuum-free self-powered parallel electron lithography with sub-35-nm resolution. Nano Letters 10, 2197-2201 (2010).
• Y. Lu, A. Lal, High-efficiency ordered silicon nano-conical-frustum array solar cells by self-powered parallel electron lithography. Nano Letters 10, 4651-4656 (2010).

5. High-efficiency nano-structured thin film solar cells

Nanostructured silicon thin film solar cells are promising, due to the strongly enhanced light trapping, high carrier collection efficiency, and potential low cost. Ordered nanostructure arrays, with large-area controllable spacing, orientation, and size, are critical for reliable light-trapping and high-efficiency solar cells. Available top-down lithography approaches to fabricate large-area ordered nanostructure arrays are challenging due to the requirement of both high lithography resolution and high throughput. Here, a novel ordered silicon nano-conical-frustum array structure, exhibiting an impressive absorbance over broad band wavelengths by a thickness of only 5 μm, is realized by our recently reported technique radioisotope-powered parallel electron lithography. Moreover, high-efficiency (up to 10.8%) solar cells are demonstrated, using these ordered ultrathin silicon nano-conical-frustum arrays.

Although these nano-structure arrays have high light absorption efficiency, the solar cell efficiency is still less than half of that for bulk crystalline Si solar cells and the open circuit voltage and fill factor are lower too. This is probably due to the high surface recombination losses, which are introduced by the highly enhanced surface area from the nanostructures, although a thin passivation oxide layer was used. Therefore, how to reduce the surface loss through appropriate surface passivation is critical to further enhance the conversion efficiency. In addition, it will be also interesting to transfer the related fabrication techniques to low-cost substrate.

Recommended references:

• J. Zhu et al., Optical absorption enhancement in amorphous silicon nanowire and nanocone    arrays. Nano Letters 9, 279-282 (2008).
• Y. Lu, A. Lal, High-efficiency ordered silicon nano-conical-frustum array solar cells by self-powered parallel electron lithography. Nano Letters 10, 4651-4656 (2010).
• J. Oh, H.C. Yuan, H. M. Branz, An 18.2%-efficient black-silicon solar cell achieved through control of carrier recombination in nanostructures. Nature Nanotechnology 7, 743-748 (2012).
• K. X. Wang, Z. Yu, V. Liu, Y. Cui, S. Fan, Absorption enhancement in ultrathin crystalline silicon solar cells with antireflection and light-trapping nanocone gratings. Nano Letters 12, 1616-1619 (2012).

6. High-efficiency betavoltaic cells by novel semiconductor nano-materials

Betavoltaics are generators that convert energy from a radioactive beta source to electricity. A beta radioisotope atom gives off a beta high energy particle (e.g., electron) through beta decay, which is governed by Poisson statistics emission event. Radioactive decay is independent of temperature and pressure. Only near nuclear fusion temperatures can the fundamental decay processes be modified. Furthermore the half-lives of many radioisotopes are several years to decades to even centuries. When the high energy beta particles traverse a semiconductor, electron-hole pairs are produced and separated by the built in electrical field of the pn junction, leading to energy conversion. Betavoltaic power sources have very promising applications in harsh environments where long life of the energy source is required. Novel semiconductor nano-materials, such as porous silicon, GaN nano-crystals, etc, have great potential to realize high efficiency betavoltaic cells.

Recommended references:

• M. V. S. Chandrashekhar, C. I. Thomas, H. Li, M. G. Spencer, A. Lal, Demonstration of a 4H sic betavoltaic cell. Applied Physics Letters 88, 033506 (2006).
• G. Hang, L. Hui, A. Lal, J. Blanchard, Nuclear microbatteries for micro and nano Devices, Proceedings of  9th International Conference on Solid-State and Integrated-Circuit Technology,pp. 2365-2370 (2008).
• G. Hang, Y. Hui, Z. Ying, Proceedings of IEEE 20th International Conference on Micro Electro    Mechanical Systems, pp. 867-870, 2007.
• T. Wacharasindhu, J. W. Kwon, D. E. Meier, J. D. Robertson, Radioisotope microbattery based   on liquid semiconductor. Applied Physics Letters 95, 014103 (2009).