About

Dr. Christos Dimitrakopoulos is a Professor in the Department of Chemical and Biomolecular Engineering at the University of Massachusetts, Amherst. He received his Ph.D., M.Phil., and M.S. degrees in Materials Science from Columbia University in 1993, and his Diploma degree from the National Technical University, Athens, Greece in 1986. His research focuses on the growth, transfer, characterization, and applications of large-area, 2D materials such as graphene and phosphorene. He works on graphene and its metamaterials for novel mechanical, optoelectronic, and thermoelectric applications, as well as microfluidics and nanocomposites. His interests also include organic semiconductors and perovskite hybrid photovoltaics and field effect transistors. Dr. Dimitrakopoulos has extensive experience in organic and hybrid semiconductor materials and devices for large area flexible electronics, wafer-scale epitaxial graphene growth, and optoelectronic devices. He previously worked at IBM T. J. Watson Research Center from 1995 to 2013, where he contributed to research on these topics and held 91 US patents. He has authored or co-authored over 90 publications with a citation count exceeding 26,550 and an h-index of 53. His work has been highly recognized, with one of his papers listed among the top-ten Most-Cited Papers in Materials Science from 1996-2006 and as the most-cited IBM paper for the decade 1998-2008. Dr. Dimitrakopoulos has given more than 70 invited talks at national and international conferences and institutions. He has received numerous honors, including being elected a Fellow of the National Academy of Inventors in December 2019 and a Senior Member in February 2019. He was awarded the Outstanding Senior Faculty Award by the College of Engineering at UMass Amherst in April 2019. His accolades also include the IEEE Paul Rappaport Award, IBM Extraordinary Technical Accomplishment Award, and multiple IBM Innovation and Division Awards. During his academic career, he received fellowships and scholarships from Columbia University and the IBM Research Fellowship. His research and professional contributions are recognized for their innovation and impact in the field of materials science and electronic materials.

Research topics

• Computer Science
• Chemistry
• Nanotechnology
• Materials science
• Chemical engineering
• Inorganic chemistry
• Engineering
• Chromatography
• Electrical engineering
• Optoelectronics
• Computer hardware

Selected publications

• Monolayer CVD Graphene Barrier Enhances the Stability of Planar p–i–n Organic–Inorganic Metal Halide Perovskite Solar Cells

  ACS Applied Energy Materials · 2021 · 8 citations

  Senior authorCorresponding
  • Materials science
  • Chemical engineering
  • Nanotechnology

  The poor environmental stability of hybrid perovskite solar cells (PSCs) remains one of the leading obstacles to their commercialization. Herein, we develop and use, for the first time, an orthogonal solvent-assisted process to transfer sheets of monolayer chemical vapor deposited (CVD) graphene onto the perovskite active layer without causing damage to the perovskite layer. We show that at this location in a standard methylammonium lead iodide PSC stack, the CVD graphene acts as a barrier layer to improve stability by (i) preventing moisture ingress into the perovskite layer and (ii) blocking the diffusion of silver ions from the electrode to the perovskite layer. Upon exposure to humidity for 1 week, unencapsulated devices with a graphene barrier retained 93% of their initial PCE, whereas those devices without a graphene barrier retained only 46%. Similarly, after heat treatment, unencapsulated devices with a graphene barrier showed no decrease in PCE, whereas those without a graphene barrier decreased to ∼75% of their initial PCE. CVD graphene is shown to be a prime candidate for improving the environmental stability of PSCs.

  PublisherDOI

• Spray-Coated, Volatile and Nonvolatile, Two-Terminal, Resistive Switching Memory Devices Comprising Liquid-Exfoliated Black Phosphorus and Graphene Layers

  IEEE Transactions on Electron Devices · 2020 · 6 citations

  Senior authorCorresponding
  • Computer Science
  • Materials science
  • Optoelectronics

  Resistive switching memory devices fabricated with black phosphorus inks showed volatile memory device characteristics, specifically static random access memory (SRAM) and bipolar resistive switching. A high ON-/OFF-current ratio of 6.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> was obtained at a reading voltage of 0.5 V with good retention stability (over 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> s). Multilevel data storage performance under different compliance currents was demonstrated. Importantly, a nonvolatile memory device was also fabricated, using an ink that comprised thinner, on average, black phosphorus flakes with a narrower thickness distribution than the ink used in the above described devices. The nonvolatile memory device showed good write/erase operation, like flash memory, during 100 endurance cycles, and good retention stability with 1.9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> of on/off ratio at 0.5 V. According to these results, we suggest that thin films of liquid-exfoliated black phosphorus deposited by spray coating are suitable for low-cost, solution-processed, two-terminal resistive memory devices, either volatile or nonvolatile ones, the memory device type being controlled by the details of the ink preparation process, and the resulting flake thickness distributions.

  PublisherDOI

• Evaporation-Induced Self-Assembly of Semi-Crystalline PbI2(DMSO) Complex Films as a Facile Route to Reproducible and Efficient Planar p-i-n Perovskite Solar Cells

  MRS Advances · 2018-01-30 · 2 citations

  articleSenior authorCorresponding
  PublisherDOI

• A Graphene-Based Microfluidic Platform for Electrocrystallization and In Situ X-ray Diffraction

  Crystals · 2018-02-01 · 20 citations

  articleOpen access

  Here, we describe a novel microfluidic platform for use in electrocrystallization experiments. The device incorporates ultra-thin graphene-based films as electrodes and as X-ray transparent windows to enable in situ X-ray diffraction analysis. Furthermore, large-area graphene films serve as a gas barrier, creating a stable sample environment over time. We characterize different methods for fabricating graphene electrodes, and validate the electrical capabilities of our device through the use of methyl viologen, a redox-sensitive dye. Proof-of-concept electrocrystallization experiments using an internal electric field at constant potential were performed using hen egg-white lysozyme (HEWL) as a model system. We observed faster nucleation and crystal growth, as well as a higher signal-to-noise for diffraction data obtained from crystals prepared in the presence of an applied electric field. Although this work is focused on the electrocrystallization of proteins for structural biology, we anticipate that this technology should also find utility in a broad range of both X-ray technologies and other applications of microfluidic technology.

  PublisherOA PDFDOI

• N-Doped Zwitterionic Fullerenes as Interlayers in Organic and Perovskite Photovoltaic Devices

  ACS Energy Letters · 2017-04-03 · 30 citations

  articleOpen access

  The efficient operation of polymer- and perovskite-based photovoltaic devices depends on selective charge extraction layers that are placed between the active layer and electrodes. Herein, we demonstrate that integration of a tetra-n-butyl ammonium iodide-doped zwitterionic fulleropyrrolidine derivative, C60-SB, as a cathode modification interlayer significantly improves the photovoltaic device performance. Compared to the intrinsic (undoped) zwitterionic material, which is an efficient interlayer itself, the doped interlayers further improve average power conversion efficiencies from 8.37% to 9.68% in polymer-based devices and from 12.53% to 15.31% in perovskite-based devices. Ultraviolet photoelectron spectroscopy revealed that doping increases the interfacial dipole at the C60-SB/Ag interface, i.e., reduces the effective work function of the resultant composite cathode. This effect originates from the population of negative polaron states in C60-SB by extrinsic charges that prevent directional charge transfer from Ag to the integer charge-transfer states in C60-SB, pinning the Fermi level at higher energy. The reduced resistivity of the doped interlayer, as measured by impedance spectroscopy, enables efficient device operation with a broad range of interlayer thicknesses, thus simplifying the solution-based device fabrication process.

  PublisherDOI

• Enhanced Quality CVD-Grown Graphene via a Double-Plateau Copper Surface Planarization Methodology

  Crystal Growth & Design · 2017-10-02 · 10 citations

  articleSenior author

  Two-dimensional (2D) nanomaterials have been of intense interest in recent years because of their exceptional electronic, thermal, and mechanical properties. Tailoring these novel properties toward their intrinsic potential requires precise control of the atomic layer growth process and the underlying catalytic growth substrate, as the morphology and purity of the catalytic surface plays a critical role on the shape, size, and growth kinetics of the 2D nanomaterial. In this work, we present a systematic study on the role of the catalytic surface morphology and interface properties on the subsequent carrier mobility properties of CVD-grown graphene. A modified electropolishing methodology results in a dramatic reduction of over 99% in Cu surface roughness that enhances the carrier mobility of the CVD-grown graphene by as much as 125% compared to unpolished and lower planarization level growth substrates, providing a clear correlation between the smoothness of the Cu growth substrate and the resulting electrical properties of the graphene. Mobility measurements also reveal a systematic and controllable reduction in carrier concentration for increased electropolishing time. In addition to enhanced transport properties, the 100-fold reduction in the copper surface roughness leads to the ability to grow high-quality graphene at lower process temperatures.

  PublisherDOI

• A novel thin-film blue light emitting diode via GaN-on-graphene technology

  2017-10-01

  article

  Fully functional thin-film blue LED was fabricated by novel means of (1) performing epitaxial growth of a single crystalline InGaN/GaN heterostructure on a recycled graphene/SiC substrate (2) followed by release and transfer of the heterostructure.

  PublisherDOI

• Graphene-based microfluidics for serial crystallography

  Lab on a Chip · 2016-01-01 · 62 citations

  articleOpen access

  Microfluidic strategies to enable the growth and subsequent serial crystallographic analysis of micro-crystals have the potential to facilitate both structural characterization and dynamic structural studies of protein targets that have been resistant to single-crystal strategies. However, adapting microfluidic crystallization platforms for micro-crystallography requires a dramatic decrease in the overall device thickness. We report a robust strategy for the straightforward incorporation of single-layer graphene into ultra-thin microfluidic devices. This architecture allows for a total material thickness of only ∼1 μm, facilitating on-chip X-ray diffraction analysis while creating a sample environment that is stable against significant water loss over several weeks. We demonstrate excellent signal-to-noise in our X-ray diffraction measurements using a 1.5 μs polychromatic X-ray exposure, and validate our approach via on-chip structure determination using hen egg white lysozyme (HEWL) as a model system. Although this work is focused on the use of graphene for protein crystallography, we anticipate that this technology should find utility in a wide range of both X-ray and other lab on a chip applications.

  PublisherOA PDFDOI

• A robust molecular probe for Ångstrom-scale analytics in liquids

  Nature Communications · 2016-08-12 · 4 citations

  articleOpen access

  Traditionally, nanomaterial profiling using a single-molecule-terminated scanning probe is performed at the vacuum-solid interface often at a few Kelvin, but is not a notion immediately associated with liquid-solid interface at room temperature. Here, using a scanning tunnelling probe functionalized with a single C60 molecule stabilized in a high-density liquid, we resolve low-dimensional surface defects, atomic interfaces and capture Ångstrom-level bond-length variations in single-layer graphene and MoS2. Atom-by-atom controllable imaging contrast is demonstrated at room temperature and the electronic structure of the C60-metal probe complex within the encompassing liquid molecules is clarified using density functional theory. Our findings demonstrates that operating a robust single-molecular probe is not restricted to ultra-high vacuum and cryogenic settings. Hence the scope of high-precision analytics can be extended towards resolving sub-molecular features of organic elements and gauging ambient compatibility of emerging layered materials with atomic-scale sensitivity under experimentally less stringent conditions.

  PublisherOA PDFDOI

• Current-driven nanowire formation on surfaces of crystalline conducting substrates

  Applied Physics Letters · 2016-05-09 · 20 citations

  articleOpen access

  The formation and precise manipulation of nanoscale features by controlling macroscopic forces is essential to advancing nanotechnology. Toward this end, we report here a theoretical study on formation of nanowires with precisely controlled widths, starting from single-layer conducting islands on crystalline conducting substrates under the controlled action of macroscopic forcing provided by an externally applied electric field that drives island edge electromigration. Numerical simulations based on an experimentally validated model and supported by linear stability theory show that large-size islands undergo a current-induced fingering instability, leading to nanowire formation after finger growth. Depending on the substrate surface crystallographic orientation, necking instabilities after fingering lead to the formation of multiple parallel nanowires per island. In all cases, the axis of the formed nanowires is aligned with the direction of the externally applied electric field. The nanowires have constant widths, on the order of 10 nm, which can be tuned by controlling the externally applied electric field strength. Our findings have important implications for developing future lithography-free nanofabrication and nanoelectronic patterning techniques.

  PublisherOA PDFDOI

Frequent coauthors

• Yuxi Wang

  Xinxiang Medical University

  31 shared

• Shuo Sui

  Cornell University

  30 shared

• Sarah L. Perry

  30 shared

• A. Grill

  Micron (United States)

  28 shared

• Phaedon Avouris

  25 shared

• Junpei Okada

  Tohoku University

  25 shared

• H. Koizumi

  Hiroshima University

  25 shared

• A. Rodríguez-Romero

  Universidad Autónoma de la Ciudad de México

  25 shared

Education

• Ph.D, M.Phil., M. Sc.

  Columbia University

  1993

Awards & honors

• Fellow of the National Academy of Inventors (NAI) (12/2019)
• Senior Member of NAI (2/2019)
• Outstanding Senior Faculty Award by the College of Engineeri…
• Paul Rappaport Award by the Institute of Electrical and Elec…
• Extraordinary Technical Accomplishment Award by IBM Corp. (2…