nanosurface 2007-10-30 22:18
石墨片找到失蹤的π了
[color=Blue]【纳米科技世界快讯】從石墨薄片(graphene)於2004年首度被發現後，「失蹤的π之謎」始終困惑著物理學家。最近，美國研究人員聲稱他們已經解開這道謎題。所謂的π 之謎是指理論預測的石墨薄片電導率與實驗量測值相差約3.14倍。該研究團隊外加電壓於長寬比不一的微小石墨薄片上，求出電導率與外加電壓及長寬比的函數關係。結果顯示理論預測無誤，但只適用於非常小且具有某些長寬比的石墨薄片。[/color]
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石墨薄片由於質地堅硬、容易製作，又具有極佳的電、熱傳導性，因此成為熱門的奈米材料研究對象。而石墨薄片只有一個原子厚的特性，也使它成為研究二維電子奇異特性的理想系統。不過，石墨薄片最奇特的性質，也許是它同時表現出金屬與半導體的行為。
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如果在石墨薄片的兩端接上電極，並加上一橫跨平面的閘極電壓，則在不同閘極電壓下會量到不同的電導率，就像半導體一般。但是半導體在閘極電壓低於某個臨界值時，電導率會降至零，石墨薄片則不然，因此表現又像導體。以往物理學家曾嘗試測量電導率的最小值，卻發現測量值比理論值大了約3.14倍。
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正當某些科學家擔心理論可能出錯時，其他人開始懷疑最小電導值是否也與石墨薄片的尺寸及外形有關。最近，加州大學河濱分校的Chun Ning Lau等人證實了這個猜測。該團隊測量了14組長度及寬度介於300至8000 nm的長方形石墨薄片。實驗數據顯示，當石墨薄片的長度小於500 nm時，如果寬度大於長度的2倍，最小電導率接近理論預測值，而若寬度小於該值，電導率便高過理論值。該團隊也發現只要長度大於3000 nm，就算寬度超過長度兩倍以上，測量值總是比理論值來的大。:O0yK s"C]j A2LT
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Lau指出，實驗結果顯示理論只適用於非常微小的石墨薄片，而且最小電導率與石墨薄片的外形有關。詳細內容請參考Science 317, p.1530 (2007)。8k0] @0j*Ij7s8^
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[b]Experiment finds graphene’s missing pi[/b]
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Physicists in the US claim to have solved the “mystery of the missing pi”, which has confounded physicists studying graphene since the material was discovered in 2004. The pi in question is related to a discrepancy between the conductance of graphene as measured in the lab and the value predicted by theory. The team measured the conductance along tiny pieces of graphene as a function of both an applied voltage and the ratio between the length and width of the sheets. The results suggest that the theory is correct -- but only for very small sheets with specific shapes (Science 317 1530). vGL#q$J
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Graphene is the darling of nanotechnologists because it is tough, easy to make and a very good conductor of both heat and electricity. The fact that it is one atom thick also makes it an ideal system for exploring the often bizarre properties of “two-dimensional” electrons.
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Perhaps the most curious property of graphene is that it appears to behave like both a metal and a semiconductor. If electrodes are placed at either end of a sheet and a gate voltage is applied across the surface, the electrical conductance along the sheet will be different for different values of the gate voltage -- just like a semiconductor. But unlike a semiconductor the conductance does not go to zero when the gate voltage drops below a certain value -- something that you would expect of a metal. In the past when physicists have tried to measure this minimum conductance, however, they have found that it is a factor of pi (about 3.14) greater than predicted by theory.
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While some worried that the theory could be wrong, others began to wonder if the minimum value was also related to the size and shape of the graphene sheet. Now, Chun Ning Lau and colleagues at the University of California at Riverside have showed this to be the case.D-@ F
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The team measured the minimum conductance of 14 different graphene rectangles with widths and lengths in the 300 to 8000 nm range. For samples with lengths smaller than 500 nm, the team discovered that the minimum conductance approached the theoretical value when the width of the rectangle was more than twice its length. However, when the width became any smaller than this, the conductance rose beyond the theoretical value. The team also found that in sheets longer than about 3000 nm, the conductance was always greater than the theoretical value, even when the width was greater than twice the length.
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Lau told physicsworld.com that the experiment shows that the theory only applies to very small pieces of graphene, and that the minimum conductance is dependent upon the shape of the sheet.
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According to Carlo Beenakker, a theoretical physicist at Leiden University in the Netherlands who studies graphene, Lau’s data agree with theoretical predictions regarding the relationship between conductance and the width and length of the sample. He told physicsworld.com that Lau’s work “closes a chapter on graphene.”VFM B4[)x7J
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Source: Physicsworld.comC,K3i"|-v$K4Y
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xuzhenhe 2008-05-28 12:24
:readrules :readrules :readrules