Experimental Physics at UVM SPS
Whereas theoretical physicists deal with abstract equations to learn more about the universe around them, experimental physicists deal with the raw data from that very universe. Experimental physicists test the theories that are formulated by the theoretical physicist, and either verifies or disproves the theory based upon empirical evidence. Experimental physics has been made famous by such historical experiments as the Cavendish experiment and the research currently taking place at the Large Hadron Collider in Geneva, Switzerland.
At UVM, experimental physics is roughly split into three categories: condensed matter, acoustic, and biological. Condensed matter physics focuses on the physical properties of condensed phases of matter (such as magneto-optical spectroscopy and thin-film growth), while acoustic physics deals with the study of mechanical waves through matter and biological physics utilizes physical experimentation to study biological systems.
Below, you will find some experimental research topics which SPS members are currently or have been engaged in.
Spin-polarized Magneto-optical Spectroscopy Studies of Nitride Semiconductors
Adviser: Prof. Madalina Furis
As we advance into the twentieth century, one of the most exciting new forms of technology can be found in organic materials. Not only are organic devices greener to use than their inorganic counterparts, but they are also much cheaper to make. At UVM, the Furis group is particularly interested in small-molecule organic thin films, which may be utilized in the fabrication of organic thin film transistors or even novel magnetic devices. To be specific, they try to better understand these organic thin films by looking at complex systems of organic molecules that self-assemble into macroscopically-ordered structures. To understand these molecules, the Furis group uses polarized light spectroscopy imaging in high magnetic fields.
To understand the experimental techniques utilized in the Furis group's studies of organic thin films, one must first understand the concept of spin. Put simply, spin is a quantum mechanical property of subatomic particles--an intrinsic form of angular momentum that makes the particle (in this case, an electron) act like a bar magnet. Now, when a special kind of polarized light (known as right circularly polarized light or RPC) is incident on a semiconductor, not all the electrons in the semiconductor will respond--only those of one kind of spin. These electrons will then free themselves from the valence band of the atom, and form a spin-polarized electric current in the semiconductor. Because the spin of the electrons make the particles act like bar magnets, we can control the direction of the spin vector by utilizing an external magnetic field, a phenomenon known as spin procession. By using optical and magnetic phenomena as described above, the Furis group can then produce net spin populations in organic semiconductors which is not parallel to the external magnetization, which could help us develop a spin transistor out of these organic materials.
The Furis group has a long tradition of engaging excited SPS students in experimental research. Two former SPS treasurers, Matthew DiMario and Cody Lamarche, both were researchers in the Furis group, and now have moved in to graduate school at the University of New Mexico and Cornell University, respectively. To learn more about the Furis group, click here to visit her webpage
Fabrication of Organic Photovoltaic Cells
Adviser: Prof. Randall Headrick
Like Dr. Furis, Dr. Headrick looks at organic materials. Indeed, they both look at organic thin films. However, whereas Dr. Furis' research revolves around the attempted fabrication of spin transistors, the Headrick group tries to fabricate organic photovoltaic cells. Modern photovoltaic fabrication, as conducted in the Headrick group, has become geared towards organic materials due to the fact that the optical absorption coefficient for some materials, as well as the fact that such cells are generally cheap to make and may be easily allow for chemical change.
To fabricate the photovoltaic cells, we first look at organic semiconductors. A semiconductor is a photovoltaic cell if it exhibits the photovoltaic effect. Closely related to the photoelectric effect, the photovoltaic effect is a process by which two different materials create a voltage when struck by energy. This might be illustrated in a classic solar cell, where the electrons freed by the external energy may easily cross the junction between the two materials, thus giving one side of the junction a negative charge and the other side a positive charge. The resulting current may then be used to power an electric circuit. To be specific, there are four main layers to an organic photovoltaic cell: the cathode, the accepter layer, the donor layer, and the anode. There are then four main steps in the photovoltaic effect that take place inside the cell:
1) First, we shine sunlight on the photoactive layers of the organic photovoltaic, i.e. the donor and accepter layers of our cell. Incident light is then absorbed by these layers, which then generates an exciton (a bound electron and hole pair). This electrons then goes from the HOMO (highest occupied molecular orbital) to the LUMO (lowest unoccupied molecular orbital).
2) An exciton (i.e, an electron-hole pair), now freed from the donor-acceptor conglomerate, diffuses to the interface of the donor-acceptor.
3) With the exciton at the interface of donor and acceptor, our electron-hole pair now splits, with the electron moving on the side of the acceptor and the hole on the side of the donor. The electrons thus move to the LUMO.
4) The electron and hole, now separated, drift to the anode and cathode, respectively. The holes of the cathode will then want to be with the electrons of the anode, and thus a current will flow in the connecting wire.
Students in the Headrick group (like ex-SPS members T.J. Howard and Evan Laird) fabricate organic photovoltaics by producing thin films. This is done by depositing the films in a glass slide in what is known as a bell jar evaporator, which is a device which generates a powerful vacuum and high temperature. Once the material has been depositing on the slide, the efficiency of the photovoltaic can then be tested. To learn more about Dr. Headrick's research, click here to visit his website.
Last modified August 12 2014 10:01 PM