User:Robert.camp.inorganic/sandbox



Stephen Maldonado is an American chemist. He is an assistant professor at the University of Michigan, specializing in electrochemistry and, in particular, semiconductor photoelectrochemistry. He is a member of the American Chemical Society and the Society for Electroanalytical Chemistry, and he has served as the section secretary of the Detroit chapter of the Electrochemical Society.

Early life and education
Maldonado was raised in Knoxville, Iowa and attended the University of Iowa, where he earned a Bachelor of Science degree in chemistry in 2001. During his undergraduate studies, he developed a strong interest in electrochemistry; he pursued this interest further during his graduate studies at the University of Texas, Austin, where he earned his Ph.D. in chemistry in 2006. He earned his degree under the supervision of Dr. Keith J. Stevenson. His thesis recounted his work with the preparation and characterization of nitrogen-doping carbon nanotube electrode materials.

Academic and professional career
After obtaining his doctorate, Maldonado worked as a research consultant for Next Dimension Technologies, Inc., a company specializing in chemical sensors and detectors, in Pasadena, California. In 2008, Maldonado obtained a postdoctoral fellowship at the California Institute of Technology, where he was supervised by Dr. Nate Lewis and gained expertise in semiconductors and the chemistry of interfaces. In the same year, Maldonado was appointed an assistant professor in the Department of Chemistry of the University of Michigan. He continues in this position, focusing in his research in the general fields of nanomaterials synthesis; photoelectrochemistry; and surface chemistry.

Carbon electrode materials
The general focus of Maldonado's doctoral work at the University of Texas was carbon nanofiber electrodes. Specifically, Maldonado generally worked on the development and characterization of nitrogen-doped carbon electrode materials. Certain advances made during his doctoral study include the synthesis of carbon nanofiber electrodes from the pyrolysis of iron(II) phthalocyanine on nickel substrates. These electrodes showed substantial electrocatalytic activity in neutral and basic solution without any significant pretreatment or surface activation. In addition, these electrodes strongly adsorbed reactive intermediates in the reduction of dioxygen to hydrogen peroxide. The presence of residual nitrogen functionalities on the surface of the carbon electrode was determined by spectroscopy to have an influence on the adsorption of reactive intermediates to the surface, thus enhancing the reaction rate.

The effect of nitrogen groups on the rate of electrocatalysis was further explored using doping experiments. Trials using the oxygen reduction reaction run on carbon nanofiber electrodes, one doped with nitrogen (N-doped) and one nondoped, showed that the N-doped electrode demonstrated electrocatalysis rates one hundred times larger than that on the nondoped electrode. Other work on carbon nanotubes directly showed that a change in the amount of nitrogen doping could affect the reactivity of the nanotubes by influencing the number of reducing sites, thus introducing a new way to control the electrocatalytic ability of the electrode with relative fineness. .

Semiconductors and interface chemistry
Much of Maldonado's work during his postdoctoral study at the California Institute of Technology concerned semiconductor chemistry. One major project investigated charge transfer at the interface of a mercury surface and either a p-type or n-type silicon surface. It was found that the functionalization of the semiconductor surface strongly affected the energy barrier heights at junctions. The silicon surface was terminated with hydrogen, various silicon oxides, and alkyl chains. In the case of a hydrogen-terminated silicon surface, the barrier height was predicted from the work function of the metal and the electron affinity of the semiconductor, while the barrier height for CH3-terminated Si could be predicted from the electron affinity of the silicon and the newly-present dipole from the alkyl group. Oxide-terminated silicon demonstrated barrier heights consistent with Fermi level pinning. These results highlight that barrier heights can be controlled with surface functionalization, building on Maldonado's earlier work with nitrogen functionalization of carbon surfaces. Corollary to this were results that showed that CH3-terminated n-Si(111) surfaces possessed a significantly lower number of nucleation sites than H-doped Si, and that electrodeposition on the former required a much larger applied voltage than the latter. The key result of this work was that methyl group functionalization of silicon allowed for significant passivation of the semiconductor surface, thus helping to reduce charge recombination, a common problem in photovoltaic devices.

Further work in this field concerned the development of n-Si/metal diodes using gold nanoparticles as precursors. The use of gold as a support for metal films prevented typical problems of metal/semiconductor interfacial reactivity from occurring.

Chemical sensors
Another major topic of Maldonado's postdoctoral work was the design of efficient vapor sensors. Among his achievements in this field is the design of sensors using carbon black and metallophthalocyanines. The resulting sensors showed large differential resistance changes when exposed to different vapors. In addition, tests using ammonia showed that the device had a sensitivity comparable to commercial sensors already on the market and maintained it for significant periods of time with no decline in sensitivity. Additional experiments with gold nanoparticles coated with straight-chain alkanethiols revealed that these structures were also sensitive to various vapors ; in these experiments, the sensitivity to most vapors decreased with increasing alkanethiol chain length.

Synthesis of nanomaterials
A general area of the study of nanomaterials in need of improvement is the synthesis of crystalline semiconductors with high aspect ratio, such as semiconductor wires. A key ongoing activity in Maldonado's laboratory at the University of Michigan is the development of efficient methods of electrodeposition of particles onto a surface to grow crystalline semiconductors. The target method of deposition is the electrochemical liquid-liquid-solid (ec-LLS) technique, in which a liquid metal electrode in a liquid can reduce dissolved oxidized precursors as well as act as a solvent from which dissolved reduced species can precipitate out. This method has allowed for the electrodeposition of crystalline semiconductors that had not previously been thought able to be deposited.

One of the goals of this work is to use ec-LLS to design electrochemical devices under conditions that are not extreme. Multiple successful examples of this use of ec-LLS have been reported by the Maldonado lab. For example, germanium has been electrodeposited successfully from germanium dioxide (GeO2) solution, dissolved in a liquid mercury electrode, and crystallized successfully. The critical advantages of this method are that the experiments were successful under conditions easily achievable on the lab bench, eliminating the need for extreme energy input, and that the rate of dissolution of germanium into the electrode is affected by the applied voltage, affording an additional measure of control to the experimenter. Similar work successfully generated germanium nanowires on scattered indium nanoparticles, and another set of experiments resulted in the formation of crystalline gallium arsenide (GaAs) from arsenic oxide on the surface of a liquid gallium electrode. Additional work allowed crystalline silicon to be formed from silicon tetrachloride (SiCl4) on a liquid gallium electrode. All of these examples were successfully run at relatively low temperatures (80°C or less), thus eliminating the need for extreme environmental conditions in the generation of crystalline semiconductors using the ec-LLS method.

Future work will focus on the development of a quantitative model to predict the conditions and outcomes of the ec-LLS process.

Photoelectrochemistry
Another focus of Maldonado's work at the University of Michigan is the development of high-efficiency photoelectrodes. It is hoped that simulations and experimental data can be combined to elucidate design parameters that will maximize the efficiency of an electrode or a solar cell.

One finding by the group was that increasing the relative amount of nanostructure in a gallium phosphide photoelectrode increased the efficiency of the cell substantially. Gallium phosphide, a material normally noted for poor charge carrier diffusion, was made more porous, resulting in a significant increase in the energy conversion efficiency of the cell in which the material was found.

Gallium phosphide was also found to have an increased efficiency when annealed in flowing ammonia. In the presence of ammonia, the gallium phosphide became alloyed with nitrogen, instead of pure gallium nitride. The nanowires that resulted from this alloying showed an increased absorbance of light and increased quantum efficiency at longer wavelengths. Thus annealing of semiconductors in ammonia or another gas can enhance the photoefficiency of a cell significantly.

New materials and material-building techniques have been developed in a search for more efficiently-generated and photoactive materials. One example of a new photoactive material is zinc germanium diphosphide (ZnGeP2), on silicon supports, which was found to be photoactive under white light. This work also highlighted a synthesis method for other films of the group type II-IV-V, which might also be photoactive. In addition, a solar cell made using polycrystalline silicon, along with silver nanoparticles for absorption and photocurrent enhancement; the efficiency was increased due to antireflective mechanisms and increased light capture due to the system geometry.

Surface chemistry
The final major area of current work in the Maldonado lab revolves around the development of new methods of changing and monitoring the surface characteristics of semiconductors, because the determination of these properties is important for understanding reactions that can occur on the surface. Ultimately, the development of methods of very fine modification of a semiconductor surface can lead to the subsequent creation of of finely-tuned electrode materials.

One of the methods used in this line of research has been wet etching using various reagents. In the case of gallium phosphide, the use of different substances in the etching of the semiconductor resulted in semiconductors with very different chemical properties. Even the different sides of a single semiconductor (one with only gallium bonding orbitals available, and the other with only phosphorus bonding orbitals available) showed vastly different properties from each other, with the phosphorus-bonding side covered with residual oxide from the etching material. It is thus apparent that wet etching with different reagents is a valuable technique for the fine-tuning of the characteristics of a semiconductor, expanding the chemical possibilities of the surface. Wet etching has also been conducted using a Grignard reagent, in the cases of gallium phosphide, gallium arsenide, and gallium nitride. The key finding of the Grignard reagent work is that such reagents are tools that allow for the relatively easy functionalization of a semiconductor surface without using thiol/sulfide chemistry or gas-phase pretreatment, a finding that will simplify the chemistry and the preparation time involved in controlling a semiconductor surface to possess certain properties.

An additional development in this area by the Maldonado group is the use of the overlayer surface-enhanced Raman spectroscopy (SERS) strategy to monitor changes in the properties of a semiconductor surface in real time. In the case of germanium on a support, Raman spectroscopy was successfully used to monitor electrodeposition conditions, the possible presence of unfilled gaps on the substrate surface, and the potential difference at which GeH formation might have occurred. The fineness of the monitoring ability afforded by SERS will allow for better control of experimental conditions to achieve a desired result.

Notable alumni

 * Dr. Michelle Chitambar, researcher at Spider 9 Dynamic Energy Systems
 * Dr. Kevin Hagedorn, researcher at IMRA America, Inc.

Honors

 * Young Investigator Award, Society of Electroanalytical Chemistry, 2014
 * Camille Dreyfus Teacher-Scholar Award, 2013
 * Alfred P. Sloan Research Fellow, 2013
 * National Science Foundation CAREER Award, 2011 - 2016
 * Moore Foundation Postdoctoral Fellowship, 2007 - 2008
 * Ford Foundation Postdoctoral Fellowship, 2006 - 2007
 * Donald D. Harrington Graduate Student Fellowship, 2003 - 2006
 * Welch Foundation Graduate Research Fellowship, 2002
 * College of Natural Sciences Graduate Fellowship, 2001 - 2002
 * National Science Foundation Graduate Student Fellowship, 2001 - 2003
 * Sanxay Prize, University of Iowa, 2001
 * Beckman Scholar Fellowship, 2000 – 2001

Service

 * Secretary of the Detroit Chapter of the Electrochemical Society, 2011-2012
 * Member of State of Michigan Green Chemistry Roundtable, 2009-2012
 * Michigan Green Chemistry Conference Session Organizer, 2009-2011
 * Pittcon 2010 and 2012 Session Organizer, 2010 and 2012
 * Member of U.S. delegation at the inaugural meeting of the Chemical Sciences and Society Symposium, 2009