Our group is interested in low dimensional nano-structured materials (such as 2D TMDC, graphene, carbon nanotubes, organic molecules) for nanoelectronic and optoelectronic device applications. The primary focus is in the fabrication of prototype devices using state of the art nano-scale patterning and lithography techniques, understanding the fundamental charge transport properties at different temperature regimes and exploiting those properties for novel applications. Current research interest includes:

1. Tailoring the properties of two dimensional (2D) transition metal dichalcogenides (TMDC):

The ability to create and manipulating defects constitutes an essential element in tailoring the electrical, magnetic, and optical properties of the host material. Altering the properties of bulk semiconductors by defect engineering has been successfully used for the development of many novel applications. Although the role of defects is well understood in bulk semiconductors, it has received little attention in emerging two-dimensional (2D) layered semiconductors, preventing their full exploitation for tailored 2D nanoelectronic and optical devices. Our group is conducting research in tailoring the properties of 2D TMDC via controlled defect engineering.

Ph.D position available: This is a new research direction and we are looking for highly motivated graduate students who are keen on contributing many excellent publications. The students will receive training on state of the art nano-scale device fabrication techniques including high resolution electron beam lithography, room temperature and low temperature electronic transport measurements and opto-electronic properties measurements. Past graduate students from my group went to industry such as Triquint semiconductors as well as in prestigious academic institutions such as Purdue University and University of Minnesota. Many of these students published more than 10 papers each in high-impact factor paper journals.

Undergraduate research position available: We are also looking for talented and highly motivated undergraduate student volunteers who are seeking research challenges and would like to publish papers in high impact factor journals.

Related publications:

1.      Muhammad R. Islam, Narae Kang, Udai Bhanu, Hari P. Paudel, Mikhail Erementchouk, Laurene Tetard, Michael N. Leuenberger, and Saiful I. Khondaker. Electrical property tuning via defect engineering of single layer MoS2 by oxygen plasma.Nanoscale 6, 10033 (2014).

2.      Udai Bhanu, Muhammad R. Islam, Laurene Tetard, and Saiful I. Khondaker, Photoluminescence quenching in gold - MoS2 hybrid nanoflakes. Scientific Reports 4, 5575 (2014). 

3.      Narae Kang, Hari P. Paudel, Michael N. Leuenberger, Laurene Tetard, and Saiful I. Khondaker, Photoluminescence quenching in Single-layer MoS2 via Oxygen Plasma Treatment. J. Phys. Chem. C (2014), DOI: 10.1021/jp506964m.

2. Graphene: Large scale assembly and tailoring properties

Graphene, the two dimensional single layer of carbon atoms arranged in a hexagonal and honeycomb structure, has attracted a great deal of attention because of its unique electronic properties making them model systems for the observation of novel quantum phenomenon and building blocks for future nanoelectronic devices. Graphene can be produced in large quantities and processed in a form of solution thru chemical exfoliation called graphene oxide sheets. We have developed multidisciplinary research effort on these materials with a strong emphasis on both fundamental science and device applications.

Related publications (selected):

1. Daeha Joung and Saiful I. Khondaker,  Two to one dimensional crossover in graphene quantum dot arrays observed in reduced graphene oxide nanoribbons. Phys. Rev. B 89, 245411 (2014)
2. Daeha Joung and Saiful I. Khondaker, Structural evolution of reduced graphene oxide with varying carbon sp2 fractions investigated via Coulomb blockade transport, J. Phys. Chem. C. 117, 26776 (2013).
3. Soumen Das, Sanjay Singh, Virendra Singh, Daeha Joung, Janet M. Dowding, Lei Zhai, Saiful I. Khondaker, William T. Self and Sudipta Seal, Oxygenated Functional Group Density on Graphene Oxide: Its Effect on Cell Toxicity, Particle and Particle systems Characterization 30, 148 (2013).
4. Daeha Joung and Saiful I. Khondaker, Efros-Shklovskii variable range hopping in reduced graphene oxide sheets of varying carbon sp2 fraction, Physical Review B, 86, 235423 (2012). 
5. Daeha Joung, Virendra Singh, Sanghoon Park, Alfons Schulte, Sudipta Seal, and Saiful I. Khondaker, Anchoring ceria nanoparticles on reduced graphene oxide and their electronic transport properties. J. Phys. Chem. C,  115, 24494–24500 (2011) 
6. Virendra Singh, Daeha Joung, Lei Zhai, Soumen Das, Saiful I. Khondaker, and Sudipta Seal. Graphene Based Materials: Past, Present and Future.  Prog Mater Sci vol 56, 1178 (2011).
7. Daeha Joung, Lei Zhai, and Saiful I. Khondaker, Coulomb blockade and hopping conduction in graphene quantum dots array,  Phys. Rev. B 83, 115323 (2011). 
8. Anindarupa Chunder, Tanusri Pal, Saiful I. Khondaker, and Lei Zhai,  Reduced Graphene Oxide/Copper Phthalocyanine Composite and Its Optoelectrical Properties, J. Phys. Chem. C, 2010, 114 (35), 15129–15135

9.     Daeha Joung, A. Chunder, Lei Zhai and Saiful I. Khondaker,   Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets. Applied Physics Letters 97, 093105 (2010).  

10.  Surajit Ghosh, Biddut K. Sarker, Anindarupa Chunder, Lei Zhai, and Saiful I. Khondaker,  Position dependent photodetector from large area reduced graphene oxide thin films. Applied Physics Letters 96, 163109 (2010).

11.  Daeha Joung,  A. Chunder, Lei Zhai, and Saiful I. Khondaker, High yield fabrication of chemically reduced graphene oxide field effect transistors by dielectrophoresis. Nnotechnology 21, 165202 (2010).

3. Carbon nanotube devices: Fabrication and device properties

 We are using high quality solution processed SWNTs in combination with AC dielectrophoresis (DEP) to fabricate SWNT field effect transistors (FETs) and single electron transistors (SETs) with high yield and examining their detailed electronic transport properties at room temperature as well as at low temperatures.

 Related publications:

1. Biddut K. Sarker, Narae Kang and Saiful I. Khondaker, High Performance Semiconducting Enriched Carbon Nanotube Thin Film Transistors Using Metallic Carbon Nanotube Electrode. Nanoscale 6, 4896 (2014). 
2. Mohammad R. Islam and Saiful I. Khondaker, Recent progress in parallel fabrication of individual single walled carbon nanotube devices using dielectrophoresis (Invited Review). Materials Express 4, 263 (2014) 
3. Paul Stokes, Mohammad R. Islam and Saiful I. Khondaker, Low temperature electron transport spectroscopy of mechanically templated carbon nanotube single electron transistors, Journal of Applied Physics vol 114, 084311 (2013).
4. Muhammad R. Islam, Kristy J. Kormondy (*), Eliot Silbar (*), and Saiful I. Khondaker, A general approach for high yield fabrication of CMOS compatible all semiconducting carbon nanotube field effect transistors. Nanotechnology 23, 125201 (2012). )  (* denotes undergraduate students)
5. Shashank Shekhar, Helge Heinrich, and Saiful I. Khondaker, Huge Volume Expansion and Structural Transformation of Carbon Nanotube Aligned Arrays during Electrical Breakdown in Vacuum. CARBON 50 (2012)  1635-1643. 
6. Kristy J. Kormondy(*), Paul Stokes, and Saiful I. Khondaker, High yield assembly and electron transport investigation of semiconducting-rich local-gated single-walled carbon nanotube field effect transistors, Nanotechnology  22, 415201 (2011)  (* denotes undergraduate student)
7. Biddut K. Sarker, Shashank Shekhar and Saiful I. Khondaker, Semiconducting enriched carbon nanotube align arrays of tunable density and their electrical transport properties. ACS Nano 5, 6297 (2011). 
8. Shashank Shekhar, Mikhail Erementchouk, Michael N. Leuenberger, and Saiful I. Khondaker, Correlated breakdown of carbon nanotubes in an ultra-high density aligned array, Applied Physics Letters, vol 98, 243121 (2011).
9. Shashank Shekhar, Paul Stokes, and Saiful I. Khondaker, Ultra-high density alignment of carbon nanotubes array by dielectrophoresis, ACS Nano, vol 5, 1739 (2011).
10. Paul Stokes and Saiful I. Khondaker, Directed assembly of solution processed single walled carbon nanotubes via dielectrophoresis: from aligned array to individual nanotube devices,  J. Vac. Sci. Technol. B 28, C6B7 (2010)
11. Paul Stokes and Saiful I. Khondaker, Evaluating defects in solution processed carbon nanotube devices via low temperature transport spectroscopy, ACS Nano Vol 4, 2659 (2010)
12. Paul Stokes and Saiful I. Khondaker, High quality solution processed carbon nanotube transistors assembled by AC dielectrophoresis, Applied Physics Letters 96, 083110 (2010)
13. Biddut K. Sarker, M. Arif, and Saiful I. Khondaker, Near-infrared photoresponse in single walled carbon nanotube/polymer composite films, CARBON 48, 1539 (2010). 
14. Biddut K. Sarker, M. Arif, Paul Stokes, and Saiful I. Khondaker, Diffusion mediated photoconduction in multi-walled carbon nanotube film. Journal of Applied Physics 106, 074307 (2009). 
15. Paul Stokes, Eliot Silbar(*), Yashira M. Zayas(*) and Saiful I. Khondaker, Solution processed large area field effect transistors from dielectrophoreticly aligned arrays of single-walled carbon nanotubes. Appl. Phys. Lett. 94, 113104 (2009)  (* denotes undergraduate students).
16. Paul Stokes, Liwei Liu, Jianhua Zou, Lei Zhai, Qun Huo and Saiful I. Khondaker, Photoresponse in large area multi-walled carbon nanotube/ polymer nanocomposite films. Appl. Phys. Lett. 94, 042110 (2009)
17. Jianhua Zou, Saiful I. Khondaker, Lei Zhai and Qun Huo, A General Strategy to Disperse and Functionalize Carbon Nanotubes Using Conjugated Block Copolymers, Advance Functional Materials 19, 479 (2009)
18. Paul Stokes and Saiful I. Khondaker, Controlled fabrication of single electron transistors from single-walled carbon nanotubes, Appl. Phys. Lett. 92, 262107 (2008).
19. J. Zou, L. Liu, H. Chen, S. I. Khondaker, R. D. McCullough, Q. Huo, and L. Zhai, Dispersion of Pristine Carbon Nanotubes Using Conjugated Block Copolymers. Advanced Materials 20, 2055 (2008).
20. Paul Stokes and Saiful I. Khondaker, Local-gated single-walled carbon nanotube field effect transistors assembled by AC dielectrophoresis, Nanotechnology 19, 175202 (2008). This article has been chosen as a feature article and has also been featured in nanotechweb.org.

4. High performance organic electronic devices using CNT and graphene electrodes

A major challenge in improving organic field effect transistor (OFET) device performance is to reduce the large interfacial barrier between metal and organic semiconductor (OSC), which results in a low charge injection from the metal electrode to OSC and limit the performance of OFETs. The large interface barrier can be originated from discontinuity in morphology, interfacial dipole barrier, and Schottky barrier. In order to address the challenge of low charge injection, carbon nanotubes and graphene has been suggested as a promising electrode material for OFETs due to its high work function and strong π-π interaction with organic molecules which can reduce the injection barrier at the electrode/organic interface. We have been working towards that endeavor for the last several years.

Related publications:

1. Biddut K. Sarker and Saiful I. Khondaker, Lower Activation Energy in Organic Field Effect Transistors with Carbon Nanotube Contact. Solid State Electronics 99, 55 (2014). 
2. Narae Kang, Biddut K. Sarker, and Saiful I. Khondaker. The Effect of Carbon Nanotube/Organic Semiconductor Interfacial Area on the Performance of Organic Transistors, Applied Physics Letters, 101, 233302 (2012).
3. Biddut K. Sarker, and Saiful I. Khondaker, Thermionic Emission and Tunneling at Carbon Nanotube-Organic Semiconductor Interface. ACS Nano, vol 6, 4993 (2012)
4. Biddut K. Sarker, and Saiful I. Khondaker, High Performance Short Channel Organic Transistors using Densely Aligned Carbon Nanotube Array Electrodes, Appl. Phys. Lett. 100, 023301 (2012)
5. Biddut K. Sarker, Jianhua Liu, Lei Zhai, and Saiful I. Khondaker, Fabrication of Organic Field Effect Transistor by Directly Grown Poly(3 Hexylthiophene) Crystalline Nanowires on Carbon Nanotube Aligned Array Electrode, ACS Appl. Mater. Interfaces 2011, 3, 1180–1185.
6. Biddut K. Sarker, Muhammad R. Islam, Feras Alzubi, and Saiful I. Khondaker, Fabrication of Aligned Carbon Nanotube Array Electrodes for Organic Electronics Devices, Mater. Express 1, 80-85 (2011)