For the discovery ofphotoluminescence in porous silicon对于发现在多孔硅的光致发光Leigh T. CanhamChief Scientific Officer, pSiMedica Ltd.,Malvern, and Honorary Professor,School of Physics and Astronomy,University of Birmingham, Birmingham, England, UKNOBEL PRIZE 2012 Nominee2012 年诺贝尔物理学奖被提名人
Luminescence (cold light, annealing) : It’s ability to emit lightwaves by solid states.Generated another reason than heating.There is a few kind of luminescence e.g. Photoluminescence (PL),electroluminiescence (EL).•PL – exited by photons beam.•EL - exited by electric field
SILICON- 硅Is the 2ndmost abundant element in the earths crust. ( 第二最丰富的元素 )Never occurs as a free element( 自由元素 ), it is always mixed with an other element.When mixed with oxygen it forms silica which is usually found as sand.Only 2 elements make up almost 3/4of the Earths crustUsed when turning solar energy into electricity.Used to make concrete an/or brick.Silica( 二氧化硅 ) is whats used to make glass.
Porous Silicon- 多孔硅• Porous silicon was discovered by accident in 1956 by Arthur Uhlir Jr. andIngeborg Uhlir at the Bell Labs in the U.S .• At the time, the Ulhirs were in the process of developing a technique forpolishing and shaping the surfaces of silicon and germanium.• In 1980s Leigh Canham reasoned that the porous silicon may display quantumconfinement effects. The intuition was followed by successful experimentalresults published in the 1990. In the published experiment, it was revealed thatsilicon wafers can emit light if subjected to electrochemical and chemicaldissolution.• Canham is suggested as a possible Nobel Prizewinner “for the discovery of photoluminescencein porous silicon”
• Porous silicon was discovered by accident. It was produced by non-uniform etching during theelectropolishing of silicon with an electrolyte containing hydrofluoric acid.• The etching resulted in a system of disordered pores with nanocrystals remaining in theinter-pore regions. Porous silicon is still manufactured by Anodization ( 阳 极 氧 化 ) orelectrochemical etching( 电化学蚀刻 )of silicon in hydrofluoric acid (HF) solutions. AqueousHF is unsuitable for the etching process because the silicon surface is hydrophobic.• The porous layer can be made more structurally uniform if an ethanolic solution( 乙醇溶液 )is used - this increases the wettability of the silicon and allows better surface penetration bythe acid. Ethanoic etch solutions also reduce the formation of hydrogen gas bubbles as ethanolacts as a surfactant and prevents bubbles sticking to the silicon surface.
Scheme of produce PSPlatinum cathode and Silicon Anode
Porous Silicon as Explosive多孔硅爆炸In 2001, a team of scientists discovered that hydrogenated porous siliconreacts explosively with oxygen at cryogenic temperatures, releasingseveral times as much energy as an equivalent amount of TNT, at amuch greater speed在 2001 年，一队科学家发现氢化多孔硅爆炸性反应在低温下与氧气，释放出尽可能多的能量等量的 TNT 几次，在一个更大的速度Explosion occurs because the oxygen, which is in a liquid state at thenecessary temperatures, is able to oxidize through the porous molecularstructure of the silicon extremely rapidly, causing a very quick andefficient detonation.发生爆炸的氧气，这是在必要的温度下以液体状态，因为能够非常迅速地通过多孔氧化硅的分子结构，从而导致一个非常快速和高效的爆轰。Although hydrogenated porous silicon would probably not be effectiveas a weapon, due to its functioning only at low temperatures, other usesare being explored for its explosive properties, such as providing thrustfor satellites.虽然氢化多孔硅可能不会有效的武器，由于其运作只能在低温，其他用途正在探索其爆炸性能，如提供卫星推力。
Porous Silicon 多孔硅In the most basic sense,porous silicon is a network ofair holes within aninterconnected silicon matrix.The size of these air holes,called pores, can vary from afew nanometers( 纳米 )to a fewmicrons( 微米 ) depending onthe conditions of formation andthe characteristics of thesilicon.The SEM(ScaningElectron Microscope) imagetypical porous silicon sample.
Pour SizeThe porosity value of silicon is a macroscopic parameter and doesn’t yield anyinformation regarding the microstructure of the layer. It is proposed that theproperties of a sample are more accurately predicted if the pore size and itsdistribution within the sample can be obtained. Therefore, porous silicon has beendivided into three categories based on the size of its pores;1.Macroporous 孔2.Mesoporous 介孔3.Microporous 微孔
Cross-sectional electron microscope imageof a porous silicon sample containing two distinct pore morphologies.The morphology is controlled by the current applied during etching.In this sample, the current was decreased suddenly during preparation,resulting in the abrupt decrease in pore diameter observed.Sample courtesy Manuel Orosco, University of California, San Diego.Electron micrograph courtesy Melanie L. Oakes, Hitachi Chemical ResearchCenter, Irvine, CA. Inset is 590 nm in diameter.
Porosity 多孔性Porosity or void fraction is a measure of the void (i.e., "empty") spacesin a material, and is a fraction of the volume of voids over the totalvolume, between 0–1, or as a percentage between 0–100%孔隙率或空隙比是衡量的空隙（即，“空”）空间中的材料，是超过总体积的空隙的体积的一小部分，在 0-1 之间，或在 0-100 ％之间的百分比The porosity of porous silicon may range from 4% for macroporouslayers to 95% for mesoporous layers.It was also found that a silicon wafer with medium to low porositydisplayed more stability.
The silicon nanocrystals inPS that emits visible light vary insize from 10-15Å (diameter).Raman spectroscopy( 拉曼光谱 )gives indirect information aboutthe microstructure of PS and hasshown that the nanocrystals alterthe selection rules relating to theinteraction of optical phononswith incident photons.Whichresult in the MORE EFFICIENTPhotoliminance andElectroluminance of PorourSilicon as compared to the Non-porous silicon.
The nanoporous structures have dimensions in the low nm-range. If the structure size reaches a value below, say 3 nm, quantumeffects can occur and therefore nanoporous samples can exhibitstrong visible photoluminescence and electroluminscence, as can beseen in the picture below.Photoluminescence of a nanoporous silicon sample
SEM images and spectra of porous Si samples.The images are examples of a low porosity 低孔隙度(left) and high porosity (middle). The spectra (right)indicate the fluorescence tunability of porous Si.
Photoluminescent properties ofporous silicon filmsPorous silicon samples with photoluminescent peaks indifferent parts of the visible rangeElectroluminescent spectrum ofporous silicon filmTypical photoluminescent spectrum ofporous silicon film
Structural color from electrochemically fabricated porous silicon photonic crystals. Theirporous nanostructures reflect specific wavelengths of light. The optical spectrum is sensitive tothe refractive index of any molecules filling the pores, which allows them to be used aschemical and biological sensors
Anodization 阳极氧化1- Silicon Wafer – P-Type, 1–10Ω cm resistivity2- Platinum Cathode( 鉑陰極 )– 99.5% pure3- Hydrofluoric Acid( 氫氟酸 )– 50%4- Ethanol( 乙醇 )5- Electrolysis CellElectrolyte solution = HF:H2O:C2H5OH = 1:1:2Current density = 19 mA cm-2Electrolysis Time = 5 minutes
Different Parameters ( 不同的參數)Electrolyte Solution 電解液 :HF:H2O:C2H5OHorHF: AGNO3orHF:HNO3:H2O:C2H5OHCurrent = DC or AC(DC used for more thin homogeneous porosity layer)Electrolysis Duration( 電解時間 )= 5 Minute to 45 MinutesBy changing the different parameters, we can get differentkinds of Porosity for Silicon.
Measurement of porosityBy changing the different parameters, we can get differentkinds of Porosity for Silicon.P = (mw-mps) / (mw-mrps)mw= Mass of wafer before Anodizationmps = Mass of wafer after Anodizationmrps = Mass of wafer after removing part of porous siliconlayer in 1N KOH SOLUTION.
Surface modification of porous silicon 多孔矽表面改性Freshly etched porous silicon may be unstable due to the rate of its oxidation( 氧化 )by the atmosphere or unsuitable for cell attachment purposes.It can be surface modified to improve stabilityThermal Oxidation: ( 熱氧化 )The process involves heating the silicon to a temperature above 1000 C to promotefull oxidation of siliconThermal Oxidation can make the pSi most stable
Properties of pSi Sample• Photoluminescence Properties(PL)PL measurements were performed at room temperature using a Si detector.The 442 nm line of a He–Cd laser and unfocused laser power of 24.5 mWwere used as an excitation source for photoluminescence. Standard lock-intechniques were used to maximize the signal-to-noise ratios, and all PLspectra were corrected for system response.