利用场发射电子显微镜研究具有原子分辨的单壁碳纳米管的场发射

利用场发射电子显微镜(FEM)研究了组装在金属钨(W)针尖上的单壁碳纳米管(SWCNTs)的电子发射特性. 透射电子显微镜(TEM)结果表明在W针尖上存在一单壁碳纳米管束. 在适当的实验条件下得到了一组具有原子分辨率的FEM像, 该像可能是从单壁碳纳米管束中突出的一根(16,0)锯齿型单壁碳纳米管开口端的发射形成的. 理论计算了FEM图像放大倍数和分辨率. 当压缩因子β的值设定为β= 1.76时, FEM图像放大倍数的理论计算值与实际测量值相一致. 利用考虑镜像力的Gomer分辨率公式计算了FEM的分辨本领. 在典型电场强度ε = 5.0×10⁷ V/cm)情况下, FEM的分辨本领可以达到0.277 nm, 这接近(16,0)型单壁碳纳米管开口端碳原子沿锯齿边的间距(0.246 nm). 这些进一步支持了实验得到的结果.     

Field emission patterns with atomic resolution of single-walled carbon nanotubes by field emission microscopy

Electron emission properties of single-walled carbon nanotubes (SWCNTs) assembled on a tungsten tip were investigated using field emission microscopy (FEM). The transmission electron microscopy (TEM) micrograph confirmed the existence of an SWCNT bundle on the W tip. Under appropriate experimental conditions,a series of FEM patterns with atomic resolution were obtained. These patterns arose possibly from the field emission of the open end of an individual (16,0) SWCNT protruding from the SWCNT bundle. The magnification factor and the resolution under our experimental conditions were calculated theoretically. If the value of the compression factor β was set at β= 1.76, the calculated value of the magnification factor was in agreement with the measured value. The resolving powerof FEM was determined by the resolution equation given by Gomer. The resolutionof 0.277 nm could be achieved under the typical electric field of 5.0×107 V/cm, which was close to the interatomic separation 0.246 nm between carbon atoms along the zigzag edge at the open end for the (16, 0) SWCNT. Consequently, our experimental results were further supported by our theoretical calculation.     

用场发射显微镜研究单壁碳纳米管场发射

利用场发射显微镜在小于40×10^-7Pa压强条件下研究了单壁碳纳米管微束在不同温度加热除气后的场发射变化,得出随着除气温度的升高,碳纳米管逸出功经历了从大到小再变大的变化过程.在清洁态(1000℃除气)对应有最小的逸出功,此时场发射图像显示出一定的细致结构.继续升高温度,碳纳米管发生塌缩.     

Al-C60-Al分子结电子输运特性的第一性原理计算

利用基于密度泛函理论的格林函数方法, 计算了Al-C60-Al分子结的电子输运特性. 考虑了C60分子在铝电极表面的原子结构弛豫, 计算结果表明共振传导是Al-C60-Al分子结电子输运的主要特征, 在费米能级附近的电导约为1.14G0 (G0=2e2/h). 投影态密度(PDOS)分析表明, Al-C60-Al分子结的电子输运主要通过C60分子的最低空分子轨道(LUMO)和次低空分子轨道(LUMO+1)进行. 讨论了C60分子和铝电极之间距离的变化对其电子输运特性的影响.     

碳纳米管端口的场蒸发

将单根多壁碳纳米管(multi-walled carbon nanotube,MWCNT)组装在W针尖上并送入超高真空场发射/场离子显微镜(Ultrahigh Vacuum Field-emission/Field-Ion microscope,UHV-FEM/FIM)进行场蒸发及场发射研究.结果表明,场蒸发可以降低MWCNT的逸出功,从而增强其场发射能力.估算MWCNT的蒸发场低于1.3×10⁸V·cm^-1,且在此场强下的平均蒸发速率为9.4nm·min^-1.定性讨论了MWCNT的蒸发场大大低于C的理论值的原因.首先,通过场解吸获得的清洁端口上有较多悬挂键,平均每个C原子的配位数较小,所以升华热较低.其次,可能存在于MWCNT中的H原子会在强场下碰撞端口的C原子,使其更易蒸发.以上结果显示了利用场蒸发剪短碳纳米管从而改善其场发射特性的可行性. 关键词： 碳纳米管 场蒸发 场发射     

INVESTIGATION ON THE STRUCTURE AND ELECTRIC PROPERTIES OF BUCKY ONIONS

Bucky onions were fabricated by the DC discharge method and their behaviors and electric properties on Highly Oriented Pyrolytic Graphite (HOPG) were studied by using an atomic force microscope (AFM), a scanning tunneling microscope (STM) and a transmission electron microscope (TEM). Small-sized Bucky onions demonstrated the properties of semiconductors and as the size increased their metallicity became stronger. AFM and STM images revealed the tendency of Bucky onions to form dimers.