Past Events

Nanofluidics with two-dimensional materials
Apr 19 @ 11:00 AM – 1:00 PM

Speaker: Prof Slaven Garaj
Affiliation: NUS
Abstract Details: The curious behavior of water and ions in constrictions with dimensions comparable to the size of ions is of particular interest for many applications, including filtration membranes, single-biomolecule analysis, supercapacitors, etc. The nanofluidic behavior of such structures depends on their dimensionality: ranging from the edge-enhanced ionic current in 0D graphene nanopores [1,2], anomalous ionic flow in 1D nanotubes, to frictionless water transport in 2D graphene [3] and graphene-oxide nanochannels [4, 5].  We set to investigate ionic flow in graphene-based nanostructures, including scalable GO membranes, and model systems consisting of individual graphene channels only about 1 nm in height. By measuring mobility of a wide selection of aqueous salts ions in channels of GO membranes [5], we demonstrated that the dominant mechanisms for the ion rejection are (a) size exclusion due to compression of the ionic hydration shell in narrow channels; and (b) electrostatic repulsion due to the membrane surface charge.  Armed with the insight into the physical mechanism governing the ionic flow, we are able to engineer new membranes with decreased the ionic cut-off size and increased charge selectivity. At the end, I will present some new results leading to promising applications in desalination and electrodialysis.

[1] Garaj, S. et al. Graphene as a subnanometre trans-electrode membrane. Nature 467, 190 (2010).

[2] Garaj, S. et al. Molecule-hugging graphene nanopores. Proc Natl Acad Sci USA 110, 12192 (2013).

[3] Radha, B. et al. Molecular transport through capillaries made with atomic-scale precision. Nature 538, 222 (2016).

[4] Nair, R. R. et al. Unimpeded Permeation of Water Through Helium-Leak–Tight Graphene-Based Membranes. Science 335, 442 (2012).

[5] Hong, S. et al. Scalable Graphene-Based Membranes for Ionic Sieving with Ultrahigh Charge Selectivity. Nano Lett. 17, 728 (2017).

About the Speaker: Slaven Garaj is Assistant Professor at the Departments of Physics and of Biomedical Engineering at the National University of Singapore, as well as a member of the NUS Centre for Advanced 2D Materials and NUSNNI-Nano Core. He is also a Singapore NRF Fellow (2012).

Slaven explores nanoscale phenomena emerging at the interface of solid-state devices and soft-matter systems. He is interested in behaviour of water molecules and ions in atomic-scale confinements; control and analysis of individual biomolecules using physical methods; and electrical and structural properties of 2D materials. The research is often guided by the desire to address a real technological challenges and includes: ultra-fast, inexpensive DNA sequencing using physical methods; nanopore devices for detection, fingerprinting and sequencing of individual proteins; electrical sensors based on 2D materials; 2D materials as next-generation membranes for filtration and water desalination.

Slaven received his PhD from Swiss Federal institute of Technology Lausanne (EPFL), Switzerland, in the field of solid-state physics. He continued his research career at Harvard University, working at the intersection of nano-electronics and biophysics, particularly by developing novel methods for electrical (4th generation) DNA sequencing based on nanopores. Throughout his career, his different research projects attracted general public attention and were featured in international media and professional magazines (such as BBC News, New Scientist, Technology Review, MRS Bulletin, etc).

Ultrafast optically induced dynamics in solids: Non-linear optics on the few-fs timescale
Jul 27 @ 11:00 AM – 12:00 PM

Speaker: Dr. Michael Sejer Wismer
Affiliation: Max Plank Institute of Quantum Optics, Germany
Abstract Details: The generation of pulses as short as a few cycles at optical frequencies allows for new regimes of nonlinear optics in solid media [1,2]. Few-cycle pulses have been shown to drive currents in insulating materials with large band gaps (~ 9 eV) at electric field strengths on the order of 1 V/Ã…, without causing structural changes to the medium.
In this talk I will present results on numerical calculations of few-cycle pulses interacting with electrons in crystalline media. A 5 fs pulse tuned to the fundamental band gap in GaAs exhibits nonlinear dynamics beyond Rabi oscillations, which is due to the significant influence of intraband motion. We argue that the modulation in transition energies caused by intraband motion leads to the appearance of anharmonic resonances.
I will also present results for the optical Faraday effect, which is likewise investigated for ultrashort pulses. Circularly polarised pump pulses with field strength close to the damage threshold are numerically shown to rotate incoming UV probe pulse which would require up to 100 T for the conventional Faraday effect. In addition, pump-probe spectroscopy of the induced ellipticity is predicted to exhibit features that have not yet been measured experimentally.

[1] Observation of high-order harmonic generation in a bulk crystal, Ghimire et al., Nature Phys., 2011.

[2] Optical-field-induced current in dielectrics Agustin, Schiffrin et al., Nature, 2013

[3]Strong-Field Resonant Dynamics in Semiconductors, Wismer et al., Phys. Rev. Lett. 2016.

[4]Ultrafast optical Faraday effect in transparent solids, Wismer et al. arXiv:1612.08433.

About the Speaker: Michael Sejer Wismer
Max Plank Institute of Quantum Optics,
Hans-Kopfermann Strae 1, Garching bei Mnchen, Germany

Origins of Nonlocal Resistance in Multiterminal Graphene: Spin Hall and Valley Hall vs. Other Competing Effects
Sep 14 @ 11:00 AM – 12:00 PM

Speaker: Prof Branislav K. Nikolić
Affiliation: University of Delaware USA
Abstract Details: The recent experimental observation of nonlocal voltage, several microns away from the nominal current path, near the Dirac point (DP) in multiterminal graphene devices with adatom-induced spin-orbit coupling or in multiterminal graphene on hexagonal boron nitride (G/hBN) heterostructures has been interpreted as the result of the direct and inverse spin Hall effect (SHE) or the direct and inverse valley Hall effect (VHE), respectively [1]. However, subsequent experiments reproducing the nonlocal signal in graphene with adatoms have also demonstrated insensitivity to the applied in-plane magnetic field, thereby suggesting its disconnect with SHE physics or any other spin-related mechanism. The theoretical interpretation of nonlocal signal in G/hBN heterostructures in terms of topological valley currents carried by the Fermi sea states just beneath the gap opened in graphene due to inversion symmetry breaking does not explain the long-standing puzzle of why the highly insulating state of G/hBN is rarely observed. Furthermore, using Landauer-Büttiker (LB) theory, as a rigorous quantum transport approach employed over the past three decades to obtain observable nonlocal voltage and the corresponding nonlocal resistance, we obtained [1] zero nonlocal signal in the same geometry used in experiments (where the channel connecting the two crossbars is much longer that its width) and for the same simplistic Hamiltonian which gives (not directly observable) quantized VH conductivity characterizing topological valley currents. In this talk, I will show how to resolve these puzzles by using first-principles Hamiltonians of graphene with adatoms or G/hBN heterostructures combined with numerically exact calculations of the nonlocal resistance based on the multiterminal LB formula [2,3]. In the case of multiterminal graphene with adatoms, we find several background mechanisms which generate nonlocal resistance even when spin-orbit coupling is switched off [2]. We also proposed a specific device geometry where nonlocal resistance due to the SHE can be isolated by removing such background contributions [2]. This will be compared with the direct and inverse intrinsic SHE as the sole origin of nonlocal resistance in graphene/transition-metal-dichalcogenide heterostructures where graphene acquires homogeneous proximity spin-orbit coupling. In the case of multiterminal G/hBN heterostructure, we demonstrate [3] the key role played by the Fermi surface edge states and the corresponding edge currents (which were missed in previous theoretical analyses based on simplistic Hamiltonian) that can explain both the nonlocal resistance and metallic-like resistivity observed in experiments while being in full accord with the very recent Josephson interferometry-based imaging of the spatial profile of edge supercurrents in G/hBN wires.


A. Cresti, B. K. Nikolić, J. H. García, and S. Roche, Riv. Nuovo Cimento 39, 587 (2016).
D. V. Tuan, J. M. Marmolejo-Tejada, X. Waintal, B. K. Nikolić, S. O. Valenzuela, and S. Roche, Phys. Rev. Lett., 117, 176602 (2016).
J. M. Marmolejo-Tejada, J. H. Garcìa, P.-H. Chang, X.-L. Sheng, A. Cresti, S. Roche, and B. K. Nikolić, arXiv:1706.09361

About the Speaker: Branislav K. Nikolić is a Professor of Physics at the University of Delaware and a Senior Visiting Scientist at RIKEN Center for Emergent Matter Science in Japan. He received his Ph.D. in theoretical condensed matter physics from Stony Brook University, and B.Sc. degree from the University of Belgrade, Serbia. He was visiting Professor at the University of Regensburg, National Taiwan University and Centre de Physique Théorique de Grenoble-Alpes. His research is focused on nonequilibrium many-body quantum systems, first-principles quantum transport and high-performance computing applied to nanostructures of interest to spintronics, nanoelectronics, thermoelectrics and nano-bio interface. His most notable contributions include studies of the spin Hall effect, spin pumping and spin torque in the presence of spin-orbit coupling, decoherence of transported spins, spin-dependent shot noise, nonequilibrium electron-magnon and electron-phonon systems, topological insulator based devices for spintronic and thermoelectric applications and graphene based devices for ultrafast DNA sequencing.

Tunable optomechanical devices at the nano scale
Nov 23 @ 11:00 AM – 12:30 PM

Speaker: Guangya Zhou
Affiliation: Department of Mechanical Engineering, National University of Singapore
Abstract Details: In this talk, I will discuss tunable nanophotonic resonators integrated with on-chip nanoelectromechanical systems (NEMS). Photonic nano resonator or nano cavity has attracted much attention and becomes increasingly important to a range of nanophotonic applications, including efficient and ultra-compact lasers, nano scale wavelength-selective add/drop multiplexers, optical filters, and high-sensitive sensors. Making nanophotonic resonators tunable is attractive, as tunable nano resonators can provide not only greater flexibility in a dynamic photonic system and but also post-process compensation capability for fabrication imperfections. Tuning nanophotonic resonators with NEMS offers outstanding advantages including low power consumption, large tuning range, absence of exotic materials, and compatible with silicon micro/nano-fabrication processes. I will introduce the NEMS tuning approaches we developed for such resonators, these include: 1) Resonance tuning through cavity evanescent field perturbation using a NEMS-driven dielectric nano probe, 2) Resonance wavelength splitting/shifting/tuning of coupled nano resonators through NEMS-induced coupling strength variation, 3) Resonance tuning by resonator’s nano-deformation driven by NEMS. In addition to tunable nanophotonic devices, I will also discuss the optomechanical interactions at the nano scale. These include demonstration / measurement of significant bipolar optical gradient forces produced by two coupled photonic crystal nanobeam cavities, observation of various “optical spring” effects in coupled nanophotonic cavities where optical fields affect the resonant frequencies of nanomechanical resonators, observation of coherent optomechanical oscillations in coupled nanobeam photonic cavities with a mechanical Q factor over a million, and mechanical mode hoping effect where optomechanical oscillation switches from one mode to the other due to mode competition.

About the Speaker:  A/Prof. Guangya Zhou (Department of Mechanical Engineering, National University of Singapore) Prof. Zhou received the B.Eng. and Ph.D. degrees in optical engineering from Zhejiang University, Hangzhou, China, in 1992 and 1997, respectively. He joined the Department of Mechanical Engineering, National University of Singapore (NUS) in 2005 as an assistant professor. And from 2012, he is an associate professor at the same department. His research interests include optical MEMS scanners, MEMS spectrometers and hyperspectral imagers, optical MEMS based ultra-compact endoscope probes, silicon nanophotonics, NEMS tunable photonic crystals, and nano scale optomechanics. He has published over a hundred research papers in peer-reviewed international journals in his field. He is also the main inventor of the miniature solid tunable lens and aperture technology, which was successfully licensed to a NUS start-up company.

Molecular electronics at GHz: Rectification Ratio vs. Non-linearity
Dec 4 @ 1:00 PM

Speaker: Dr Jorge Trasobares Sanchez
Abstract Details: Here we propose a study on high frequency molecular rectifiers [1] using an array of sub-15 nm single gold crystal as a suitable test bed for Molecular Electronics [2,3]. Firstly E-beam lithography was used for versatile fabrication of the arrays [4]. Later, the molecular functionalization of the ferrocenylalkyl thiol self-assembled monolayer was corroborated by XPS analysis and Electrochemical measurements. Cyclic voltammetry measurements show two molecular organizations with signatures of cooperative effects [3], a dense and a diluted phase localized on top and facets of the nanocrystals respectively. Finally, direct current and radio frequency (RF) properties were simultaneously measured with the tip of an Interferometric Scanning Microwave Microscope. From the RF measurements, we extrapolate a cut-off frequency of 520 GHz. A comparison with the silicon RF- Schottky diodes, architecture suggests that the RF-molecular diodes are extremely attractive for scaling and high frequency operation. At the end of the discussion I will examine the importance of strong non-linearity versus the rectification ratio for applications such as RF-mixers.

[1] J. Trasobares, D. Vuillaume, D. Théron, N. Clement, A 17 GHz Molecular Rectifier, Nat.Commun. 7, 12850 (2016).

[2] N. Clement, G. Patriarche, K, Smaali, F. Vaurette, K. Nishiguchi, D. Troadec, A. Fujiwara, D. Vuillaume. Large array of sub-10-nm single-grain Au nanodots for use in nanotechnology. Small, 7, 2607 (2011).

[3] J. Trasobares, J. Rech, T. Jonckeere, T. Martin, O. Aleveque, E. Levillain, V. Diez-Cabanes, Y. Olivier, J. Cornil, J.P. Nys, R. Sivakumarasamy, K. Smaali, Ph. Leclère, A. Fujiwara, D. Théron, D. Vuillaume, N. Clément. Estimation of π-π Electronic Couplings from Current Measurements. Nano Letters, 17, 3215-3224 (2017).

[4] J. Trasobares, F. Vaurette, M. François, H. Romijn, J-L. Codron, D. Vuillaume, D. Théron and N. Clément. High speed e-beam lithography for gold nanoarray fabrication and use in nanotechnology. Beilstein J. Nanotechnol. 5, 1918–1925 (2014).

Photonics and polaritonics with van der Waals heterostructures
Dec 11 @ 11:00 AM – 12:30 PM

Speaker: Dr. Alexander Tartakovskii
Affiliation: University of Sheffield, UK
Host: Assistant Professor Goki Eda
Abstract Details: Monolayer films of van der Waals crystals of transition metal dichalcogenides (TMDs) are direct band gap semiconductors exhibiting excitons with very large binding energies and small Bohr radii, leading to a high oscillator strength of the exciton optical transition. Together with graphene as transparent electrode and hexagonal boron nitride (hBN) as an insulator, TMD monolayers can be used to produce so-called van der Waals heterostructures. Here we use this approach to make electrically pumped light-emitting quantum wells (LEQWs) [1,2] and single-photon emitters [3]. We combine this new technology with optical microcavities to demonstrate control of the emitter spectral properties and directionality, making first steps towards electrically injected TMD lasers [4]. Furthermore, by embedding MoSe2/hBN structures in tuneable microcavities, we enter the regime of the strong light-matter interaction and observe formation of exciton-polaritons [5]. Here we demonstrate that the magnitude of the characteristic anti-crossing between the cavity modes and the MoSe2 excitons (a Rabi splitting) can be enhanced by embedding a multiple-QW structure, containing two MoSe2 monolayers separated by an hBN barrier. We extend this work to demonstrate valley addressable polaritons in both MoSe2 and WSe2, the property inherited from valley excitons, but strongly modified through changes in exciton relaxation in the strong-coupling regime [6]. As the next step towards strongly interacting polaritons, we explore type-II semiconducting TMD heterostructures [7], where we observe Moire excitons and unusual optical selection rules.

[1] F. Withers et al., NATURE MATERIALS, 14, 301 (2015).
[2] F. Withers et al., NANO LETTERS, 15, 8223 (2015).
[3] S. Schwarz et al., 2D Materials, 3 (2016).
[4] S. Schwarz et al., NANO LETTERS, 14, 7003 (2014).
[5] S. Dufferwiel et al., NATURE COMMUNICATIONS, 6, 8579 (2015).
[6] S. Dufferwiel et al. , NATURE PHOTONICS 11, 497 (2017).
[7] E. M. Alexeev et al., NANO LETTERS, 17, 5342 (2017).

About the Speaker:  Alexander Tartakovskii is a Professor of Solid State Physics at the Department of Physics and Astronomy of the University of Sheffield. He graduated with a degree in Applied Physics and Math from Moscow Institute of Physics and Technology (Russia), and obtained his PhD in solid state physics from the Institute of Solid State Physics in Chernogolovka (Russia). His initial contributions to the field were in optical studies of non-linear exciton-polariton phenomena in III-V semiconductor microcavities comprising quantum wells. He moved to the University of Sheffield (UK) in 2001 as a postdoctoral researcher and worked on spin physics in semiconductor quantum dots, with particular emphasis on nuclear magnetism in nano-structures and novel solid state NMR techniques applied to extremely small nuclear spin ensembles in strained semiconductors. In 2005 he was awarded a prestigious EPSRC Advanced Research Fellowship, and in 2007 became a permanent faculty member. In the last few years he started working on optical studies of novel two-dimensional materials, reporting on some of the first realisations of light-emitting devices with electrical injection as well as exciton-polariton phenomena in monolayer semiconductors.

Drumhead surface states in topological nodal line semimetals
Dec 28 @ 11:00 AM – 12:30 PM

Speaker: Dr. Nimisha Raghuvanshi
Affiliation: POSTECH, Korea
Host: Prof. Vitor M. Pereira
Abstract Details: Nodal-line semimetals are characterized by one-dimensional nodal rings in the bulk protected by symmetry. Projection of these nodal rings onto the surface of a three dimensional topological semimetal leads to a new class of topological surface states known as drumhead surface states. Materials hosting these exotic features are expected to exhibit several quantum phenomena along with unusual transport characteristics and hence are promising candidates for device application and quantum information. Our research aims at verifying the existence and stability of the drumhead surface states in noncentrosymmetric semimetals.

About the Speaker: Research interests:
Theoretical Condensed Matter Physics
- Topological superconductors and semi-metals, Half-heusler alloys
- Magnetism in iron based superconductors
- Multi-orbital correlated itinerant models, SDW state, transverse spin fluctuations and susceptibility in broken-symmetry state up to random phase approximation, stabilization of the magnetic state and spin waves in multiorbital models for iron pnictides.

Advanced optical nanostructures for bio-photonics and neural engineering
Jan 9 @ 11:00 AM – 1:00 PM

Speaker: Dr Francesco De Angelis
Abstract Details: In the last years we introduced different 2D and 3D nanostructures and devices for managing the electromagnetic field at the nanoscales through the generation of surface plasmons polaritons. Firstly, we will briefly revise our past achievements concerning plasmonic nanostructures and their applications to bio-sensing. Secondly, we will show our recent achievements and future perspectives of plasmonic nanopores for next generation sequencing of DNA and protein (European Project FET-Open “Proseqo”, GA N°687089). In the final part we will present the exploitation of 3D nano-devices in combination with CMOS arrays for intracellular recording of action potentials in mammalian neurons and intracellular delivery of biomolecules, genic materials and nanoparticles. Also, the active interaction of the cell membrane with such 3D devices will be discussed. The developed platform may enable significant advances in the investigation of the neuronal code, development of artificial retinas and low-cost in-vitro platforms devoted to the pharmacological screening of drugs for the central nervous system. As future perspective we will also discuss potential application of our system for the investigation of electrical activities of plant roots that in the near future may revolutionize plant biology. This project is supported by the European Community through the IDEAS grant program (“Neuroplasmonics”, GA N° 616213).

About the Speaker: He is currently Senior Scientist at the Italian Institute of Technology and Supervisor of Nanostructure Facility (clean room). He leads the Plasmon technology Unit (about 25 members) and his main expertise relies on micro and nano-optical devices for biomedical applications. He currently holds an IDEAS-ERC Consolidator grant whose aim is to develop radically new interfaces between electrical/optical devices and neuronal networks. He published more than 100 papers on peer-review impacted journals; total impact factor > 700; H index = 37, citations>5000.

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Nature-inspired Electronic Sensor Skins
Jan 16 @ 11:00 AM – 12:00 PM
Nature-inspired Electronic Sensor Skins @ S16 Level 6 – Theory Common Conference Room

Speaker: Dr. Benjamin C.K. Tee
Affiliation: National University of Singapore (NUS)
Host: Prof. Vitor M. Pereira
Abstract Details: Electronic sensor skins are an active area of multi-disciplinary research for many groups over the world due to its potential to enable dramatic changes in how we interact with the digital environment. For example, ‘robots’ can don on sensor active skins to interact with the environment, shake human hands with comfortable pressure, or measure our health biometrics. In my talk, I will discuss the development of electronic sensor skins with some historical context, followed by showcasing of several force sensitive electronic skin technologies with high sensitivity, stretchability and bio-mimetic self-healing abilities. More recently, we demonstrated a power-efficient artificial mechano-receptor system inspired by biological mechano-receptors. We further used a channelrhodopsin with fast kinetics and large photocurrents as an optical interface to neuronal systems for next generation opto-tactile prosthetic interfaces.

About the Speaker: Dr. Benjamin C.K. Tee is the President’s Assistant Professor at the National University of Singapore (NUS), and staff scientist in the Institute of Materials Research and Engineering (IMRE). During his doctoral career, he developed multiple technologies in electronic sensor skins with several high impact publications in Science, Nature Materials and Nature Nanotechnology. He has won numerous international awards in recognition of his work, including the prestigious MIT TR35 Innovators Under 35 Award (Global and Asia list). He is a named inventor in 8 patents. In 2014, he was selected to be a Stanford Biodesign Global Innovation Fellow (Singapore-Stanford Biodesign). During his fellowship, he applied a needs-driven methodology to identify and develop technological solutions for unmet clinical needs.

His current research focus is on developing high-performance flexible and stretchable sensor platform technologies for emerging autonomous artificial intelligence (AI) systems and Internet of Things applications. He aims to integrate fundamental knowledge in material science, nano-electronics and biology to develop multi-scale artificial sensory devices and biotechnology systems inspired by natural systems. He recently received the prestigious Singapore Young Scientist Award and was selected as a National Research Foundation (NRF) Fellow. Contact :

Two-dimensional Transition Metal Dichalcogenides Lateral Multijunctions as Nano-Optoelectronic Platforms
Jan 31 @ 2:30 PM – 4:00 PM

Speaker: Dr. Yutsung Tsai (Center for Complex Quantum Systems in University of Texas, Austin)
Date: Wed, 31/01/2018 – 2:30pm to 4:00pm
Host:Associate Professor Shaffique Adam
Location: S16 Level 6 – Theory Common Conference Room
Event Type: Seminars


Heterostructures are artificially engineered systems that consist of two or more dissimilar semiconductor junctions. Scientists have developed many combinations of heterostructures like Si/SiGe, GaAs/AlGaAs, ZnS/CdSe, and HgTe/CdTe for high-speed electronic and optoelectronic devices by tailoring those vertical multijunctions through quantum confinement. In recent years, the emergence of transition metal dichalcogenides (TMDs) has opened new frontiers in heterostructure research. In particular, their monolayer form not only enables optoelectronic application with their direct-gap band structure, but also provides a myriad of possibilities through vertical stacking due to their van der Waals (vdW) interactions between layers. On the other hand, although vertically stacked TMDs optoelectronic heterostructures have been demonstrated extensively, atomically-thin two-dimensional (2D) TMDs lateral multijunctions beyond two heterojunctions have only been explored sparingly; this has limited the development of 2D optoelectronics.

In this talk, I would like to present recently achieved lateral 2D TMDs multijunctions about the growing method, their heterostructures, optoelectronic properties, and the photo-generated carrier transport mechanism. My motivation of developing these multijunctions as nano-optoelectronic platforms for quantum information, photovoltaics, and light-induced superconductivity will then be revealed and discussed. This critical development will be the building block for more advanced 2D optoelectronic architecture.

About the Speaker

Dr. Yutsung Tsai received his PhD in Physics from the State University of New York at Buffalo in 2015 and have been working as a postdoctoral research fellow at the Center for Complex Quantum Systems in University of Texas at Austin for the past two years. He has participated in condensed matter experimental research in seven labs since his undergraduate training and become a believer for “better collaboration makes better research.” The PhD students and undergraduate students mentored by him advocated his devotion by acquiring advanced experimental skills like atomic force microscopy, micro-Raman compact mapping and appearing as first authors of publications on Nature Nanotechnology.

His current professional interests focus on optoelectronic semiconductors, particularly two-dimensional transition metal dichalcogenides (2D TMDs) lateral multijunctions MX2/M’X’2/MX2 (M=W, Mo; X=S, Se) for photovoltaic applications. He recently observed photo-generated carriers trapped by nanoscale quantum confined structures that was manifested by 100-fold photoconductivity at M’X’2 junction. This discovery enables innovative technology, for example, harvesting these trapped photo-generated carriers for solar cell use.