Transmission electron microscopy without aberrations: Applications to materials science

Angus Kirkland,Lan-Yun Chang,Sarah Haigh,Crispin Hetherington 한국물리학회 2008 Current Applied Physics Vol.8 No.3,4

Aberration correction leads to a substantial improvement in the directly interpretable resolution of transmission electron microscopes.Direct electron optical correction based on a hexapole corrector and indirect computational analysis of a focal or tilt series of imagesoer complementary approaches and a combination of the two provides additional advantages. This paper describes aberration cor-rected instrumentation installed in Oxford which is equipped with correctors for both the image-forming and probe-forming lenses.bining direct and indirect methods.

Atomic structure and dynamics of metal dopant pairs in graphene.

He, Zhengyu,He, Kuang,Robertson, Alex W,Kirkland, Angus I,Kim, Dongwook,Ihm, Jisoon,Yoon, Euijoon,Lee, Gun-Do,Warner, Jamie H American Chemical Society 2014 NANO LETTERS Vol.14 No.7

<P>We present an atomic resolution structural study of covalently bonded dopant pairs in the lattice of monolayer graphene. Two iron (Fe) metal atoms that are covalently bonded within the graphene lattice are observed and their interaction with each other is investigated. The two metal atom dopants can form small paired clusters of varied geometry within graphene vacancy defects. The two Fe atoms are created within a 10 nm diameter predefined location in graphene by manipulating a focused electron beam (80 kV) on the surface of graphene containing an intentionally deposited Fe precursor reservoir. Aberration-corrected transmission electron microscopy at 80 kV has been used to investigate the atomic structure and real time dynamics of Fe dimers embedded in graphene vacancies. Four different stable structures have been observed; two variants of an Fe dimer in a graphene trivacancy, an Fe dimer embedded in two adjacent monovacancies and an Fe dimer trapped by a quadvacancy. According to spin-sensitive DFT calculations, these dimer structures all possess magnetic moments of either 2.00 or 4.00 μB. The dimer structures were found to evolve from an initial single Fe atom dopant trapped in a graphene vacancy.</P>

Stability and Dynamics of the Tetravacancy in Graphene

Robertson, Alex W.,Lee, Gun-Do,He, Kuang,Yoon, Euijoon,Kirkland, Angus I.,Warner, Jamie H. American Chemical Society 2014 NANO LETTERS Vol.14 No.3

<P>The relative prevalence of various configurations of the tetravacancy defect in monolayer graphene has been examined using aberration corrected transmission electron microscopy (TEM). It was found that the two most common structures are extended linear defect structures, with the 3-fold symmetric Y-tetravacancy seldom imaged, in spite of this being a low energy state. Using density functional theory and tight-binding molecular dynamics calculations, we have determined that our TEM observations support a dynamic model of the tetravacancy under electron irradiation, with Stone–Wales bond rotations providing a mechanism for defect relaxation into lowest energy configurations. The most prevalent tetravacancy structures, while not necessarily having the lowest formation energy, are found to have a local energy minimum in the overall energy landscape for tetravacancies, explaining their relatively high occurrence.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2014/nalefd.2014.14.issue-3/nl500119p/production/images/medium/nl-2014-00119p_0010.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl500119p'>ACS Electronic Supporting Info</A></P>

Bond Length and Charge Density Variations within Extended Arm Chair Defects in Graphene

Warner, Jamie H.,Lee, Gun-Do,He, Kuang,Robertson, Alex. W.,Yoon, Euijoon,Kirkland, Angus I. American Chemical Society 2013 ACS NANO Vol.7 No.11

<P>Extended linear arm chair defects are intentionally fabricated in suspended monolayer graphene using controlled focused electron beam irradiation. The atomic structure is accurately determined using aberration-corrected transmission electron microscopy with monochromation of the electron source to achieve ∼80 pm spatial resolution at an accelerating voltage of 80 kV. We show that the introduction of atomic vacancies in graphene disrupts the uniformity of C–C bond lengths immediately surrounding linear arm chair defects in graphene. The measured changes in C–C bond lengths are related to density functional theory (DFT) calculations of charge density variation and corresponding DFT calculated structural models. We show good correlation between the DFT predicted localized charge depletion and structural models with HRTEM measured bond elongation within the carbon tetragon structure of graphene. Further evidence of bond elongation within graphene defects is obtained from imaging a pair of 5-8-5 divacancies.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2013/ancac3.2013.7.issue-11/nn403517m/production/images/medium/nn-2013-03517m_0009.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn403517m'>ACS Electronic Supporting Info</A></P>

Atomic Structure and Spectroscopy of Single Metal (Cr, V) Substitutional Dopants in Monolayer MoS₂

Robertson, Alex W.,Lin, Yung-Chang,Wang, Shanshan,Sawada, Hidetaka,Allen, Christopher S.,Chen, Qu,Lee, Sungwoo,Lee, Gun-Do,Lee, Joohee,Han, Seungwu,Yoon, Euijoon,Kirkland, Angus I.,Kim, Heeyeon,Suenag American Chemical Society 2016 ACS NANO Vol.10 No.11

<P>Dopants in two-dimensional dichalcogenides have a significant role in affecting electronic, mechanical, and interfacial properties. Controllable doping is desired for the intentional modification of such properties to enhance performance; however, unwanted defects and impurity dopants also have a detrimental impact, as often found for chemical vapor deposition (CVD) grown films. The reliable identification, and subsequent characterization, of dopants is therefore of significant importance. Here, we show that Cr and V impurity atoms are found in CVD grown MoS2 monolayer 2D crystals as single atom substitutional dopants in place of Mo. We attribute these impurities to trace elements present in the MoO3 CVD precursor. Simultaneous annular dark field scanning transmission electron microscopy (ADF-STEM) and electron energy loss spectroscopy (EELS) is used to map the location of metal atom substitutions of Cr and V in MoS2 monolayers with single atom precision. The Cr and V are stable under electron irradiation at 60 to 80 kV, when incorporated into line defects, and when heated to elevated temperatures. The combined ADF-STEM and EELS differentiates these Cr and V dopants from other similar contrast defect structures, such as 2S self-interstitials at the Mo site, preventing misidentification. Density functional theory calculations reveal that the presence of Cr or V causes changes to the density of states, indicating doping of the MoS2 material. These transferred impurities could help explain the presence of trapped charges in CVD prepared MoS2.</P>

Partial Dislocations in Graphene and Their Atomic Level Migration Dynamics

Robertson, Alex W.,Lee, Gun-Do,He, Kuang,Fan, Ye,Allen, Christopher S.,Lee, Sungwoo,Kim, Heeyeon,Yoon, Euijoon,Zheng, Haimei,Kirkland, Angus I.,Warner, Jamie H. American Chemical Society 2015 NANO LETTERS Vol.15 No.9

<P>We demonstrate the formation of partial dislocations in graphene at elevated temperatures of ≥500 °C with single atom resolution aberration corrected transmission electron microscopy. The partial dislocations spatially redistribute strain in the lattice, providing an energetically more favorable configuration to the perfect dislocation. Low-energy migration paths mediated by partial dislocation formation have been observed, providing insights into the atomistic dynamics of graphene during annealing. These results are important for understanding the high temperature plasticity of graphene and partial dislocation behavior in related crystal systems, such as diamond cubic materials.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2015/nalefd.2015.15.issue-9/acs.nanolett.5b02080/production/images/medium/nl-2015-02080e_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl5b02080'>ACS Electronic Supporting Info</A></P>