柯福顺

题目

Structural and Dynamical Properties of Ag₇₄Ge₂₆ Alloy studied by Ab-initio Molecular Dynamics Simulation

摘要

       AgGe, considered as the potential metallic glass material. At the early stage, AgGe alloy quenched from the liquid or sprayed by plasma-jet is in a metastable, single phase, HCP at cooling rate estimated to be 107K/s. For a faster quenched process, the short-range structure evolution of AgGe liquid, especially supercool state, directly influence the formation of amorphous alloy. In Ag-Ge system, the eutectic composition is at 25.9% at % Ge.

       In this work, Ab-initio molecular dynamics simulations of AgGe alloy at temperatures through 1200K to 773K are performed successively. Quenching process keeps alloy in liquid state below melting point around 976K. Atom distribution states are characterized by pair-correlation function, bond-angle distribution function, HA index, Voronoi index and the atomistic cluster alignment. Our results show that the icosahedral short-range order (ISRO) enhances as the temperature descending, and the Ag attraction leads the microstructure transformation under rapid cooling.

参考文献

1. Wang, J., et al., Thermodynamic description of the Au-Ag-Ge ternary system. Thermochimica Acta, 2011. 512(1-2): p. 240-246.

2. Klement, W., R.H. Willens, and P. Duwez, NON-CRYSTALLINE STRUCTURE IN SOLIDIFIED GOLD-SILICON ALLOYS. Nature, 1960. 187(4740): p. 869-870.

3. Ananthar.Tr, H.L. Luo, and W. Klement, FORMATION OF NEW INTERMETALLIC PHASES IN BINARY EUTECTIC SYSTEMS BY DRASTIC UNDERCOOLING OF MELT. Nature, 1966. 210(5040): p. 1040-&.

4. Moss, M., D.L. Smith, and R.A. Lefever, METASTABLE PHASES + SUPERCONDUCTORS PRODUCED BY PLASMA-JET SPRAYING Applied Physics Letters, 1964. 5(6): p. 120-&.

5. Duwez, P., R.H. Willens, and W. Klement, CONTINUOUS SERIES OF METASTABLE SOLID SOLUTIONS IN SILVER-COPPER ALLOYS. Journal of Applied Physics, 1960. 31(6): p. 1136-1137.

6. Kresse, G. and J. Hafner, AB-INITIO MOLECULAR-DYNAMICS FOR OPEN-SHELL TRANSITION-METALS. Physical Review B, 1993. 48(17): p. 13115-13118.

7. Kresse, G. and J. Furthmuller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Computational Materials Science, 1996. 6(1): p. 15-50.

8. Blochl, P.E., PROJECTOR AUGMENTED-WAVE METHOD. Physical Review B, 1994. 50(24): p. 17953-17979.

9. Perdew, J.P., K. Burke, and M. Ernzerhof, Generalized gradient approximation made simple. Physical Review Letters, 1996. 77(18): p. 3865-3868.

10. Chirawatkul, P., et al., Structure of eutectic liquids in the Au-Si, Au-Ge, and Ag-Ge binary systems by neutron diffraction. Physical Review B, 2011. 83(1).

11. Waseda, Y., The structure of non-crystalline material-liquids and amorphous solids. 1981, New York: McGraw-Hill.

12. Fang, X.W., et al., Atomistic cluster alignment method for local order mining in liquids and glasses. Physical Review B, 2010. 82(18).

13. Yang, L., S. Kado, and G. Derge, SELF-DIFFUSION OF SILVER IN MOLTEN SILVER. Transactions of the American Institute of Mining and Metallurgical Engineers, 1958. 212: p. 628-630.

董晓

题目：The study of Chalcogen-doped black silicon

摘要：The contents of my report include the discovery of black silicon, the fundamental properties of black silicon, the application of black silicon, and the summary and future directions.

In 1998, the Mazur group reported the initial discovery of black silicon. The preparation of black silicon is as follows: a flat silicon surface is transformed into a forest of quasi-ordered micrometersized conical structures upon irradiation with several hundred femtosecond laser pulses in an atmosphere of sulfur hexafluoride [1, 2]. The appearance of this material is black, so we call it “black silicon”. The manufacturing process is as follows: firstly, the silicon wafer for laser exposure is cleaned using RCA method; secondly, the silicon wafer is loaded into the vacuum chamber, thirdly, the vacuum chamber is evacuated to high vacuum and a background gas is backfilled [3, 4].

The femtosecond microstructured black silicon consist of a core of undisturbed silicon covered with a highly disordered layer of silicon about several hundred nanometers thick including nanocrystallites, nanopores, and sulfur impurities [5]. The electron diffraction indicates that the core is crystalline and the surface layer is polycrystalline or microcrystalline [5].

Chalcogen-doped black silicon has significant optical properties that it can absorb the whole range of sun light (from 0.25µm to 2.5µm) [6]. The background gas has a great impact on the sub-band gap absorption. Samples irradiated in the presence of H₂S and SF₆ have flat, featureless 90% absorptance for incident radiation in this wavelength range, while samples irradiated in the presence of H₂ and SiH₄ have an absorptance that falls monotonically for wavelengths longer than 1.1 µm [7]. The numbers of pulses, laser fluence, and background gas pressure also have great impact on the sub-band gap absorption [7, 8]. From the phenomenon we can draw a conclusion that the doped atoms may play a significant role in sub-band gap absorption of black silicon. The interpretation for this interesting phenomenon is that the doped of Chalcogen atoms may introduce some impurity levels in the deep of silicon band gap [9]. With the increases of impurity concentration, impurity levels transform to impurity band, which could cause sub-band gap absorption [4, 9]. 

The interaction between the fs-laser and the surface of silicon wafer will generated a large number of defects in the silicon lattice [9]. These defects have great negative effects on the photoelectric conversion of black silicon, so an annealing process is inevitable. However, the sub-band gap absorption of black silicon decreases significantly after annealing [9-11]. From the interpretation given by Mazur group, the drop of near-infrared absorption after annealing is attributed to “Diffusion theory”[11]. The diffusion theory indicates that the infrared absorption decreases monotonically with diffusion length [11]. A probably cause of the deactivation of infrared absorption is precipitation of the dopant at the grain boundaries or precipitation of the dopant occurs within the crystalline grains via clustering of dopant atoms [11]. Our group regards this phenomenon as that the impurities states changed when annealing the sample.

This material would have great potential values in the photovoltaic field for its interesting properties [12]. Black silicon could also be used as surface-enhanced Raman spectroscopy active substrate for its special microstructure [13].

At last, I give the summary and future directions in my report.

参考文献

[1] Tsing-Hua Her,  Richard J. Finlay,  Claudia Wu, Shrenik Deliwala, and Eric Mazur, Microstructuring of silicon with femtosecond laser pulses, APPLIED PHYSICS LETTERS, VOLUME 73, NUMBER 12, 21 SEPTEMBER 1998.

[2] Brian R. Tull, James E. Carey, Eric Mazur,Joel P. McDonald, and Steven M. YalisoveSilicon SurfaceMorphologies after Femtosecond Laser Irradiation, MRS BULLETIN • VOLUME 31 • AUGUST 2006.

[3] Studying femtosecond-laser hyperdoping by controlling surface morphology, JOURNAL OF APPLIED PHYSICS 111, 093511 (2012)

[4] Non-Equilbrium Chalcogen Concentrations in Silicon: Physical Structure, Electronic Transport, and Photovoltaic Potential PhD thesis, Harvard University, 2009

[5] C. H. Crouch,a) J. E. Carey, J. M. Warrender, M. J. Aziz, and E. Mazur, F. Y. Ge´ nin, Comparison of structure and properties of femtosecond and nanosecond laser-structured silicon, Appl. Phys. Lett., Vol. 84, No. 11, 15 March 2004.

[6] Femtosecond Laser Ablation of Silicon: Nanoparticles, Doping and Photovoltaics, PhD thesis, Harvard University, 2007

[7] Michael A. Sheehy,  Luke Winston,  James E. Carey,  Cynthia M. Friend,, and Eric Mazur, Role of the Background Gas in the Morphology and Optical Properties of Laser-Microstructured Silicon, Chem. Mater. 2005, 17, 3582-3586.

[8] Infrared absorption by sulfur-doped silicon formed by femtosecond laser irradiation, Appl. Phys. A 79, 1635–1641 (2004), c.h. crouch, j.e. carey, m. shen2 e. mazur, f.y. g´enin

[9] Sulfur point defects in crystalline and amorphous silicon, PHYSICAL REVIEW B 70, 205210 (2004)

[10] Michael A. Sheehy a,1, Brian R. Tull b,∗, Cynthia M. Friend a,b,1,2, Eric Mazur, Chalcogen doping of silicon via intense femtosecond-laser irradiation, Materials Science and Engineering B 137 (2007) 289–294.

[11] Brian R. Tull Mark T. Winkler Eric Mazur, The role of diffusion in broadband infrared absorption in chalcogen-doped silicon, Appl Phys A (2009) 96: 327–334.

[12] Visible and near-infrared responsivity of femtosecond-laser microstructured silicon photodiodes, July 15, 2005 / Vol. 30, No. 14 / OPTICS LETTERS

[13] Femtosecond Laser-Nanostructured Substrates for Surface-Enhanced Raman ScatteringLangmuir, Vol. 25, No. 3, 2009 1793

董果风

题目：Single-atom manipulation by tip

摘要：During the past years,single-atom manipulation has been the subject of intense research in scientific area.The first work is done by Eigler et al in1990,which is using a STM to move and reposition single Xe atoms absorbed on Ni(110) surface at a 4K.This technique has promising future in nanoscience since,for example,it makes possible for people to fabricate designed nanostructures atom by atom.STM and AFM are the two main tools for single-atom manipulation.With the tip of a STM or AFM,single

Atoms can be manipulated laterally or vertically on surfaces.The reliability of the manipulation is naturally a key issue for single-atom manipulation.The manipulation can be divided into two area:lateral manipulation and vertical manipulation.

   In lateral manipulation,according to different motion trait,Meyer et al deduced three lateral manipulation model:pulling,sliding, pushing.By changing the tunneling resistance,we can change the mode from pulling to sliding.For Cu adatom on a Cu(111) surface,the tip-apex geometry,tip height and tip orientation can influence the reliability.For the single-atom apex tip the manipulation reliability increases monotonically with decreasing tip height.Compared to it,for the dimer and trimer apex tips,the manipulation reliability is greatly improved over a certain tip-height range.Two kinds of mechanisms are found responsible for this improvement:the so-called enhanced interaction mechanism,the suspended atom mechanism due to the strong vertical attraction of the timer apex tip on the adatom.For dimer apex tip,the second mechanism is not effective.For Pt/Pt(111),the manipulation reliability is sensitive to the tip orientation in s lower tip-height range for the single-atom apex tip,and choosing an appropriate tip orientation is important.

   In vertical manipulation,Cu adatom can realize reversible vertical manipulation on Cu(111) surface,based on its strong vertical and lateral attraction on the adatom,without any electric fields.With the trimer apex tip,the adatom absorbed near the step edge even in the step can all be picked up,and in reverse the atom can be released from the tip to the surface in a certain way.In the process,the tip position must be adjusted properly while electric fields are not necessary.All these properties can apply to Al(111) surface.Based on this,the nonoclusters on surfaces canbe modified in a reversible way on an atom-by-atom basis.As an illustration,there is an example of modifying a ten-atom hexagonal cluster to a triangle one.The MD simulations show that this process can be completed at a temperature less than or equal to 100K,which indicates that the method can be reliable against the thermal disturbance and provides a theoretical guidance fot the real experiments.

  For CO molecule,the CO is adsorbed in on-top sites and stands upright on the Cu(111) surface with the C bonding to the metal atoms.As the CO molecules have a similar adsorption geometry on isolated Cu atoms adsorbed on Cu substrate,the CO molecules have to flip around when being transferred vertically.By changing the electric field,the CO can be picked up and put down.AFM is also an important tool for atom manipulation.Woking in frequency modulating mode,the AFM can laterally moving the adatom at semiconductor surfaces,for example,the Ge(111)-c(2*8).By nanoindentation,the AFM can be used to vertically manipulate adatoms on an insulator surface,for example,KCL（100).

参考文献

Gerhard Meyer,Sven Zöphel and Karl-Heinz Riedr,1996,Physical Review Letters,77(10),2113-2116

L. Bartels, G. Meyer, and K.-H. Rieder,1997,Physical Review Letters,79(4),697-700

Ryuji Nishi, Daisuke Miyagawa et al,2006,Nanotechnology,17,S142-S147

Noriaki Oyabu,Yoshi aki Sugimoto et al,2005,Nanotechnology,16,S112-117

Gerhard Meyer et al,1999,Superlattices and Microstructures,25(1/2),463-471

S.W. Hla et al,2000,Surface Science,454-456,1079-1084

谢逸群,利用探针在金属表面进行单原子操纵的理论研究，博士学位论文，2008