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Shape Control of QDs Studied by Cross-sectional Scanning Tunneling Microscopy
J. G. Keizer,M. Bozkurt,J. Bocquel,P. M. Koenraad,T. Mano,T. Noda,K. Sakoda,E. C. Clark,M. Bichler,G. Abstreiter,J. J. Finley,W. Lu,T. Rohel,H. Folliot,N. Bertru 한국물리학회 2011 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.58 No.51
In this cross-sectional scanning tunneling microscopy study we investigated various techniques to control the shape of self-assembled quantum dots (QDs) and wetting layers (WLs). The result shows that application of an indium flush during the growth of strained InGaAs/GaAs QD layers results in flattened QDs and a reduced WL. The height of the QDs and WLs could be controlled by varying the thickness of the first capping layer. Concerning the technique of antimony capping we show that the surfactant properties of Sb result in the preservation of the shape of strained InAs/InP QDs during overgrowth. This could be achieved by both a growth interrupt under Sb flux and capping with a thin GaAsSb layer prior to overgrowth of the uncapped QDs. The technique of droplet epitaxy was investigated by a structural analysis of strain free GaAs/AlGaAs QDs. We show that the QDs have a Gaussian shape, that the WL is less than 1 bilayer thick, and that minor intermixing of Al with the QDs takes place.
Influence of Grain Boundary Character on Dopants Segregation in Nanocrystalline Aluminum
Wenye Ye,Mano Misra,Pradeep Menezes,Leslie T Mushongera 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.1
This work focuses on understanding the influence of grain boundary (GB) character on magnesium (Mg) dopants segregationin nanocrystalline (NC) aluminum (Al) using a hybrid Monte Carlo/molecular dynamics (MC/MD) framework. Studies aredone to identify the dopant segregation behavior in symmetric (Σ3) and asymmetric Σ5, Σ7, Σ9, Σ11, Σ13 and Σ61 tilt GBs. The simulations reveal that GB segregation is apparently not random, instead, dopants prefer to segregate to specific sitesin the GBs. It is found that tilt GBs exhibit periodic “V”-like cluster of atoms. The segregating Mg dopant atoms replaceonly those atoms at the foot of the “V”-clusters. The number of dopant atoms absorbed by a GB is directly correlated to thenumber of such sites in a GB. Dopant segregation to high angle GBs (HAGBs) is more preferential than to low angle GBs(LAGBs) and tilt boundaries (TBs), as more the “V”-atomic clusters are available in HAGBs. Dopants bring Σ5, Σ7, Σ9, Σ11,Σ13 and Σ61 GBs to lower energetic states, which increases the resistance of the NC material to deformation. The inabilityof Σ3 GB to accommodate dopants results in an excess of dopants in the bulk, which distorts the crystalline structure, thusreducing the resistance to deformation.
Platinum-based nanocages with subnanometer-thick walls and well-defined, controllable facets
Zhang, Lei,Roling, Luke T.,Wang, Xue,Vara, Madeline,Chi, Miaofang,Liu, Jingyue,Choi, Sang-Il,Park, Jinho,Herron, Jeffrey A.,Xie, Zhaoxiong,Mavrikakis, Manos,Xia, Younan American Association for the Advancement of Scienc 2015 Science Vol.349 No.6246
<P><B>Etching platinum nanocage catalysts</B></P><P>Although platinum is an excellent catalyst for the oxygen reduction reaction that occurs in fuel cells, its scarcity continues to drive efforts to improve its utilization. Zhang <I>et al.</I> made nanocages of platinum by coating palladium nanocrystals with only a few layers of platinum and then etching away the palladium core (see the Perspective by Strasser). Platinum nanocages made using nanoscale octahedra and cubes of palladium displayed different catalytic activity for the oxygen reduction reaction.</P><P><I>Science</I>, this issue p. 412; see also p. 379</P><P>A cost-effective catalyst should have a high dispersion of the active atoms, together with a controllable surface structure for the optimization of activity, selectivity, or both. We fabricated nanocages by depositing a few atomic layers of platinum (Pt) as conformal shells on palladium (Pd) nanocrystals with well-defined facets and then etching away the Pd templates. Density functional theory calculations suggest that the etching is initiated via a mechanism that involves the formation of vacancies through the removal of Pd atoms incorporated into the outermost layer during the deposition of Pt. With the use of Pd nanoscale cubes and octahedra as templates, we obtained Pt cubic and octahedral nanocages enclosed by {100} and {111} facets, respectively, which exhibited distinctive catalytic activities toward oxygen reduction.</P>
Park, Jinho,Zhang, Lei,Choi, Sang-Il,Roling, Luke T.,Lu, Ning,Herron, Jeffrey A.,Xie, Shuifen,Wang, Jinguo,Kim, Moon J.,Mavrikakis, Manos,Xia, Younan American Chemical Society 2015 ACS NANO Vol.9 No.3
<P>We systematically evaluated two different approaches to the syntheses of Pd@Pt<SUB><I>n</I>L</SUB> (<I>n</I> = 2–5) core–shell octahedra. We initially prepared the core–shell octahedra using a polyol-based route by titrating a Pt(IV) precursor into the growth solution containing Pd octahedral seeds at 200 °C through the use of a syringe pump. The number of Pt atomic layers could be precisely controlled from two to five by increasing the volume of the precursor solution while fixing the amount of seeds. We then demonstrated the synthesis of Pd@Pt<SUB><I>n</I>L</SUB> octahedra using a water-based route at 95 °C through the one-shot injection of a Pt(II) precursor. Due to the large difference in reaction temperature, the Pd@Pt<SUB><I>n</I>L</SUB> octahedra obtained <I>via</I> the water-based route showed sharper corners than their counterparts obtained through the polyol-based route. When compared to a commercial Pt/C catalyst based upon 3.2 nm Pt particles, the Pd@Pt<SUB><I>n</I>L</SUB> octahedra prepared using both methods showed similar remarkable enhancement in terms of activity (both specific and mass) and durability toward the oxygen reduction reaction. Calculations based upon periodic, self-consistent density functional theory suggested that the enhancement in specific activity for the Pd@Pt<SUB><I>n</I>L</SUB> octahedra could be attributed to the destabilization of OH on their Pt<SUB><I>n</I>L</SUB>*/Pd(111) surface relative to the {111} and {100} facets exposed on the surface of Pt/C. The destabilization of OH facilitates its hydrogenation, which was found to be the rate-limiting step of the oxygen reduction reaction on all these surfaces.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2015/ancac3.2015.9.issue-3/nn506387w/production/images/medium/nn-2014-06387w_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn506387w'>ACS Electronic Supporting Info</A></P>