Site: Delft University of Technology (DUT)
Faculty of Chemical Technology and Materials Science
Delft Inst. of Microelectronics & Submicron Technology
2628 AL Delft, The Netherlands
Date Visited: 16 October 1997
WTEC: D.T. Shaw (report author), H. Goronkin, M.C. Roco
The nanoparticle/nanostructured materials/nanodevices research activities at the Delft University of Technology (DUT) reside primarily in the Faculty of Chemical Technology and Materials Science (STM) and Delft Institute of Microelectronics and Submicron technology (DIMES). STM is one of the more research-oriented faculties of DUT, which is the largest, oldest, and most complete technical university in the Netherlands. About 40% of all PhD degrees awarded at the University have been in STM. Among the 3 research areas the program covers (biotechnology, chemical engineering, and materials science technology), the WTEC visit concentrated primarily on chemical engineering and materials science technology.
DIMES is a large national research center in microelectronics managed by the University. Funded by the government and accredited by the Royal Netherlands Academy of Arts and Sciences, DIMES was created in 1987 as a national facility for the fabrication of advanced integrated circuits and nanostructured devices. Its 300-odd staff members and graduate students actively explore the microelectronics field; studies range from carbon nanotube electronics to nanofabrication of single-electron tunneling devices. In addition to its vast (2,000m2) clean-room fabrication facilities, the center is also a training ground for scientific specialists in advanced microelectronics in the Netherlands. Since its inception, more than a hundred PhD students have finished their dissertations using DIMES facilities. The Institute offers some twenty graduate courses, as well as extensive training support for research students. It coordinates its graduate education programs with the University's academic faculties, including the Faculty of Chemical Technology and Materials Science.
The Faculty of Chemical Technology and Materials Science (STM)
STM has strong programs in both nanoparticle generation and functional materials synthesis. Several professors in STM are active in the organization of an European Science Foundation program on Vapor-Phase Synthesis and Processing of Nanoparticle Materials. The principal objective of the program is to promote interdisciplinary collaboration between the leading research groups in Europe in aerosol and materials science. The program, with an annual budget of about FFr. 1.5 million (for meetings, workshops, and information dissemination), is coordinated by J. Schoonman of DUT and Prof. H. Fissan of the University of Duisburg, Germany.
Two major groups are engaging in nanoparticle/nanostructured materials research. One is led by Prof. J. Schoonman and the other by Prof. B. Scarlett, who was the WTEC team's host for the visit. Prof. Scarlett has several projects in nanoparticle technology, including the production of nanoparticles by electrospraying, electrostatic charging for micromixing, and nanoparticle formation in a laser-heated aerosol reactor. His group also developed photon correlation spectroscopy as a means to measure submicron particles in a gas, in situ. His coworker, Dr. J.C.M. Marijnissen summarized recent results on the generation of metal-oxide particles by bipolar mixing techniques. Different precursors (i.e., titanium tetrabutoxide, zirconium tetrabutoxide, etc.) have been used to generate metal-oxide particles. A project in collaboration with Prof. Schoonman concerns the generation of nanoparticles by laser-induced chemical vapor precipitation (LCVP). Although the LCVP technique is not new, the project's data in particle-generation parametric optimization are useful in scale-up production of Si, SiC, Si3N4, and SiCxNy nanoparticles. The research activities of Prof. Schoonman, have focused also on the synthesis of custom-designed structural and functional nanostructural materials. For example, his coworker Dr. A. Goossens reported the use of an electrostatic spray pyrolysis technique for the deposition of dense or nanoporous ceramic thin films for lithium batteries and other energy-related applications. A great variety of metal oxide nanocrystalline thin films have been synthesized, including TiO2 and Li CoO2. STM researchers have developed a technique called laser particle precipitation-aided chemical vapor deposition (LPPCVD), which produces thin films with very low substrate temperatures. Typically, the nanoparticle size is in the range of 10 to 30 nm.
Dr. Cees Dekker (Faculty of Applied Physics) made a short presentation on research activities in the area of quantum transport through nanostructures. Prof. J.E. Mooij (who was touring the United States at the time of our visit) heads the research program supported by DIMES. Four projects were discussed briefly:
1. Junction Arrays (Project Director, Hans Mooij) and Single Electronics (Project Director, Peter Hadly). The quantum behaviors of small circuits of superconducting tunnel junctions were studied experimentally and theoretically. A quality sample was designed and fabricated, in which quantum superpositions of charge states, as well as vortex states, have been experimentally observed. Quantum vortices were studied in one-dimensional arrays. Disorder was seen to lead to localization, while in periodic superlattices the vortices maintained their mobility. Fluctuations were studied in normal metals near a tunnel barrier.
In single electronics, there are three main efforts: (1) Fabrication of small junctions for the study of charging effects, (2) understanding of high-frequency behavior of single-electron tunneling (SET) transistors, and (3) characterization of single-electron circuits. Figure B.5 shows an RS flip-flop consisting of four SET transistors, each with three gates fabricated by the group.
2. Quantum Dots (Project Director, Leo Kouwenhoven). Transport experiments on quantum dots were performed on a gated device, as shown in Figure B.6. Measurements of gate voltages vs. source-drain voltages show a shell structure corresponding to a 2-D harmonic confinement potential in normal atoms. Staff observed that the filling of a shell occurs according to Hunds rule: electrons occupying degenerate states prefer to have parallel spins, which lowers the total energy due to an increased exchange interaction.
3. Single Molecular Wires (Project Director, Cees Dekker). Single-wall carbon nanotubes were obtained from R.E. Smalley at Rice University for transport measurements. The nanotubes behave as coherent quantum wires at the single-molecular scale. The density of states appears to consist of well-separated discrete electron states. The approximate 0.4 meV energy separation corresponds to estimated 1-D quantum box where a 3 mm long nanotube constitutes the electron box. Electrical conduction through these discrete electron states appears to occur quantum coherently over micron-length distance.
For transport measurements of single metal nanoclusters, DIMES researchers showed results made on a 20 nm Pd cluster, which was trapped electrostatically between two nanometer-sized electrodes (Figure B.7).
4. NEXT Nanolithography (Project Director, Bart Geerligs). The Nanoscale Experiments and Technology Project is based on a facility for the fabrication and study of sub-10 nm structures. The project studies mesophysics on 10 nm to atomic-size structures and assesses the applicability of these mesophysical phenomena to future electronic devices. Main features characteristics of the NEXT system are: (1) working in an uninterrupted, clean, ultrahigh-vacuum environment, and (2) using maskless fabrication techniques based on scanning tunneling probes. On-going experiments in the facility include quantum electronic studies of metal quantum dots, 1-D mesoscale systems, and transport in molecules.
Figure B.5. Figure B.6. Figure B.7.
RS flip-flop of four SET transistors, each with three gates.
(a) Schematic diagram of the gated quantum dot device and (b) the Coulomb oscillations in the current vs. gate voltage at B = OT observed for a 0.5 mm diameter dot.
Single Pd colloid cluster of 20 nm diameter that has been deposited between two nanoelectrodes.
The research facilities in nanostructured materials at Delft University of Technology in general, and at DIMES in particular, are very impressive. Projects are carried out with close collaboration between researchers from industry and university. For many projects at DIMES it is common to have several industrial sponsors. Since the research is generally precommercial in nature, issues about intellectual property appear not to be a problem for these multiindustrial sponsorships.