Structural Biochemistry/Inorganic Chemistry/Semimetals

Semimetals

 * Boron (B)
 * Silicon (Si)
 * Germanium (Ge)
 * Arsenic (As)
 * Antimony (Sb)
 * Tellurium (Te)

The semimetals are parts of group 13-16 of the periodic table of the elements. The semimetals, or metalloids, resemble metals in some aspects and nonmetals in other aspects. As such, most of their properties are intermediate among the elements. For example, their electronegativities and ionization energies generally lie between those of metals and nonmetals. Other properties such as boiling points, melting points, and densities vary widely. Semimetals are especially important for their intermediate level of conductivity, which makes them the basis of semiconductors.

Introduction of Porous Silicon
Arthur Uhlir Jr. and Ingeborg Uhlir discovered porous silicon by chance in 1956. It took place at Bell Labs in the United States. Their experiments involved developing the surfaces and shapes of silicon; however, under several test trials, the silicon being tested formed unwanted colored layers on its surface, such as black and red. This mishap was ignored and went unrecognized for over 20 years.

Towards the end of the 1980's, the discovery of porous silicon was noticed by the scientific community. A research analyst named Leigh Canham commented that the discovery of porous silicon proved to have some effects and advantages to the quantam confinement. After experiments and tests were ran in the 1990's, results showed that the porous silicon was able to emit light in chemical dissolution. More interest evolved in the scientific community and a large amount of experiments were conducted throughout the rest of the 1990's.

The application of research area of Porous Silicon is actually based on Porous silicon dioxide chip, which normally has 40nm-70nm irregular holes on chip. Inside of holes, there are negative charges on the surface of oxidized porous Si. These negative charges have been used to bind protein using for medical purpose. The size of holes mainly depends on currency strength during the etching process. The resistance of silicon chip and surface area of silicon chip also affect the size of holes. The silicon chips are unstable in high pH. The pH condition higher than 7 will cause silicon chip dissolve. Silicon chip has stable and negatively charges surface between pH4~7.

Picture of porous silicon:

http://sailorgroup.ucsd.edu/research/images/pSi_xsectionTHN.JPG

Porosity
The definition of porosity is the fraction of void within the pSi layer, which can be determined by weight measurement. The porosity depends on current density, HF concentration, and thickness of silicon layer. The range of porosity of silicon chip is between 4% to 95%. The porous silicon layer is more stable in medium to low porosity condition. The formula of porosity is: P=[(V0-V)/V0 ]=[1-V/V0 ] =(1-P0 /P)×100 %

Pore size
Based on the sizes of silicon pore, porous silicon layer are divided to three categories: macroporous, mesoporous, and microporous.

macroporous is defined as pore width less than 2nm.

mesoporous is defined as pore width between 2nm and 50nm.

microporous is defined as pore width larger than 50nm.

Procedure of Etching Silicon Chip
Example for 1.3cm*1.3cm silicon chip

Picture of Etching instrument of porous silicon:

http://sailorgroup.ucsd.edu/research/images/etch_cell_schematic.gif

Preparation:

Preparing Aqueous HF (48%), Ethanol (99.9%), highly doped p-type Si chip (normally has resistivity ranging from 0.0008 to 0.001 Ω-cm), a piece of aluminum foil, and Teflon etching cell with a platinum counter electrode.

Procedure:

1, Obtaining 30ml HF and 10ml Ethanol, gently mixing the solution.

2, Cutting silicon chip into 1.3cm*1.3cm pieces, aluminum foil into 5.0cm*2.0cm piece.

3, Setting up the silicon chip in the order as picture shows and checking airtightness of the Teflon etching cell.

4, Putting the Teflon etching cell on the operation platform in etching area and adjusting position of etching ring and the Teflon etching cell.

5, Adding HF/ethanol mixing solution into the Teflon etching cell until it totally submerges etching ring.

6, Using computer software to set up time and currency strength, and then beginning to etch. (Normally researchers will etch twice times. First time only etching 5 seconds, and then rinsing chip, letting chip react with NaOH to clean the surface of silicon chip. On the second time, researchers will set up computer data as what they really want to obtain in research. As the chip introduced in the example, the currency strength that may polish chip is higher than 800mA.)

7, Rinsing silicon chip twice in ethanol and drying the chip with nitrogen gun. (In order to reduce the damage caused in evaporate process, sometimes researchers will use pentane instead of ethanol, which has higher surface tension.)

8, Using computer software Fringe to obtain the data, and saving pictures for all chips in both air and ethanol situation.

9, Oxidizing silicon chip into a tube furnace at 750 °C for 1 hour in air and then let it cool to room temperature.

Reference
1. Canham, L. T. 1993, A glowing future for silicon, New Scientist.

2.Sailor Group website: http://sailorgroup.ucsd.edu/research/porous_Si_intro.html

3. Elizabeth C. Wu, Ji-Ho Park, Jennifer Park, Ester Segal, Fre#de#rique Cunin, and Michael J. Sailor, "Oxidation-Triggered Release of Fluorescent Molecules or Drugs from Mesoporous Si Microparticles", ACS Nano, 2008, 2 (11), 2401-2409 • Publication Date (Web): 08 November 2008

4. Michelle Y. Chen and Michael J. Sailor, "Charge-Gated Transport of Proteins in Nanostructured Optical Films of Mesoporous Silica"

5. Jennifer S. Andrew, Emily J. Anglin, Elizabeth C. Wu, Michelle Y. Chen, Lingyun Cheng, William R. Freeman, and Michael J. Sailor, "Sustained Release of a Monoclonal Antibody from Electrochemically Prepared Mesoporous Silicon Oxide"

6.http://en.wikipedia.org/wiki/Porous_silicon

7.http://baike.baidu.com/view/653736.htm

8. Oxtoby, David. (2008). Principles of Modern Chemistry, 6th Ed., ISBN0-534-49366-1.