Silicene

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Structure of a typical silicene cluster showing ripples across the surface.

Silicene is a two-dimensional allotrope of silicon, similar to graphene. It may have never been fabricated.[1]

History[edit]

Although theorists had speculated about the existence and possible properties of silicene,[2][3][4] researchers first observed silicon structures that were suggestive of silicene in 2010.[5][6] Using the scanning tunneling microscope they studied self-assembled silicene nanoribbons and silicene sheets deposited onto a silver crystal, Ag(110) and Ag(111), with atomic resolution. The images revealed hexagons in a honeycomb structure similar to that of graphene. Density functional theory (DFT) calculations showed that silicon atoms tend to form such honeycomb structures on silver, and adopt a slight curvature that makes the graphene-like configuration more likely. However, such a model has been invalidated for Si/Ag(110): the Ag surface displays a missing-row reconstruction upon Si adsorption [7] and the honeycomb structures observed should be attributed to tip artefacts.[8]

Similarities and Differences with Graphene[edit]

Much like how silicon and carbon are very similar atoms coming from the same group on the periodic table but still have many different properties, their 2D structures of silicene and graphene also are quite similar but have some important differences.[9] While both form hexagonal structures, only the graphene is completely flat, while the silicene structure forms a buckled hexagonal shape.[9] The buckled structure of silicene gives it the unique property of being able to tune the band gap of the material without chemical modification. This can be done by applying an external electric field.[9] Silicene also has a much more exothermic hydrogenation reaction than graphene. Another difference is that since covalent bonds between silicon do not have pi-stacking, silicene will not cluster into a graphite-like form unlike graphene.[9]

Silicene and graphene do have similar electronic structures. Both are have a Dirac cone and linear electronic dispersion around the k point. Both also have a quantum spin Hall effect.[9] Both are expected to have the characteristics of massless Dirac fermions that carry charge, but it is only predicted for silicene and has not been observed. This is because it is expected to only occur with free-standing silicene which has not yet been synthesized. It is believed that the substance silicene is made on has a substantial effect on the electronic properties of the silicene.[9]

Tunable Band Gap[edit]

Silicene is similar to graphene in many respects, but not in all. Unlike graphene, early studies of silicene have shown that different dopants within the silicene structure provide the ability to tune the band gap of the system.[10] With a tunable band gap, specific electronic components could be made-to-order for applications that require specific band gaps. It has been shown that the band-gap of silicene can be brought down to a minimum of 0.1eV, which is considerably smaller than the band gap (0.4eV) found in traditional field effect transistors (FETs).[10]

Inducing n-type doping within the silicene structure requires an alkali metal dopant. By varying the amount of doping of alkali metals, the band gap can be tuned.[10] Maximum doping of silicene with alkali metals increases the band gap 0.5eV. Due to heavy doping, the supply voltage must also be brought up to ~30V. Alkali metal doped silicene can only produce n-type semiconductors; modern day electronics require a complimentary n-type and p-type junction. Neutral doping (i-type) of silicene is also required to produce devices such as light emitting diodes (LEDs). LEDs use a p-i-n junction to produce light. A separate dopant must be introduced to generate p-type doped silicene. Iridium (Ir) doped silicene allows p-type silicene to be created.[10] Through platinum (Pt) doping, i-type silicene is possible.[10] With the combination of n-type, p-type, and i-type doped structures, many doors are opened to provide silicene with a place in modern day electronics.

Power dissipation within traditional metal oxide semiconductor field effect transistors (MOSFETs) generates a bottleneck when dealing with nano-electronics. Tunnel field-effect transistors (TFETs) are being looked at as an alternative to traditional MOSFETs because they can have a smaller subthreshold slope and supply voltage which reduce power dissipation. Computational studies have shown that silicene based TFETs outperform traditional silicon based MOSFETs.[10] These calculations have shown that the on-state current of silicene TFETs have an on-state current over 1mA/μm, a sub-threshold slope of 77mV/dec, and a supply voltage of 1.7 V. With this much increased on-state current and reduced supply voltage, the power dissipation within these devices is far below that of traditional MOSFETs and its peer TFETs.[10]

Close up of one hexagonal ring in silicene with displayed buckled structure.

Properties[edit]

2D silicene is not fully planar, apparently featuring chair-like puckering distortions in the rings. This leads to ordered surface ripples. Hydrogenation of silicenes to silicanes is exothermic. This led to the prediction that the process of conversion of silicene to silicane (hydrogenated silicene) is a candidate for hydrogen storage. Unlike graphite, which consists of weakly held stacks of graphene layers through dispersion forces, interlayer coupling in silicenes is very strong.

The buckling of the hexagonal structure of silicene is caused by pseudo-Jahn-Teller distortion (PJT).[9] This is caused by strong vibronic coupling of unoccupied molecular orbitals (UMO) and occupied molecular orbitals (OMO). These orbitals are close enough in energy to cause the distortion to high symmetry configurations of silicene.[9] The buckled structure can be flattened by suppressing the PJT distortion by increasing the energy gap between the UMO and OMO. This can be done by adding a lithium ion to the silicene structure.[9]

In addition to its potential compatibility with existing semiconductor techniques, silicene has the advantage that its edges do not exhibit oxygen reactivity.[11]

In 2012 several groups independently reported ordered phases on the Ag(111) surface.[12][13][14] Results from angle-resolved photoemission spectroscopy (ARPES) appeared to show that silicene would have similar electronic properties as graphene, namely an electronic dispersion resembling that of relativistic Dirac fermions at the K points of the Brillouin zone,[12] but the interpretation was later disputed and shown to arise due to a substrate band.[15][16][17][18] The existence of Dirac fermions for silicene on Ag(111) was later reported from scanning tunneling spectroscopy measurements.[19]

Besides silver, silicene has been reported to grow on ZrB2,[20] and iridium.[21] Furthermore, theoretical studies have predicted that silicene is stable on the Al(111) surface as a honeycomb-structured monolayer (with a binding energy similar to that observed on the 4x4 Ag(111) surface) as well as a new form dubbed "polygonal silicene", its structure consisting of 3-, 4-, 5- and 6-sided polygons.[22]

The p-d hybridisation mechanism between Ag and Si has been shown to be important to stabilise the nearly flat silicon clusters and the effectiveness of Ag substrate for silicene growth explained by density functional theory calculations and molecular dynamics simulations.[23]

Functionalized silicene

Beyond the pure silicene structure, research into functionalized silicene has yielded successful growth of organomodified silicene - oxygen-free silicene sheets functionalized with phenyl rings.[24] Such functionalization allows for uniform dispersion of the structure in organic solvents and indicates the potential for the development of a range of new functionalized silicon systems and organosilicon nanosheets.

Density functional theory calculations and molecular dynamics simulations of these novel organomodified silicene sheets have indicated that the material remains stable with sp3 hybridization with a large (direct) band gap of 1.92 eV (much wider than that of bulk Si).[25] This indicates that silicene's electronic properties can be modified by altering the functional groups on the nanosheet. Further studies have also suggested that functionalization with phenol groups may be possible.[26]

See also[edit]

References[edit]

  1. ^ Breaux, Justin H.s. (25 July 2014). "Existence of two-dimensional nanomaterial silicene questioned". Physorg. Retrieved 27 July 2014. 
  2. ^ Takeda, K.; Shiraishi, K. (1994). "Theoretical possibility of stage corrugation in Si and Ge analogs of graphite". Physical Review B 50 (20): 14916. doi:10.1103/PhysRevB.50.14916.  edit
  3. ^ Guzmán-Verri, G.; Lew Yan Voon, L. (2007). "Electronic structure of silicon-based nanostructures". Physical Review B 76 (7). doi:10.1103/PhysRevB.76.075131.  edit
  4. ^ Cahangirov, S.; Topsakal, M.; Aktürk, E.; Şahin, H.; Ciraci, S. (2009). "Two- and One-Dimensional Honeycomb Structures of Silicon and Germanium". Physical Review Letters 102 (23). doi:10.1103/PhysRevLett.102.236804.  edit
  5. ^ Aufray, B.; Kara, A.; Vizzini, S. B.; Oughaddou, H.; LéAndri, C.; Ealet, B.; Le Lay, G. (2010). "Graphene-like silicon nanoribbons on Ag(110): A possible formation of silicene". Applied Physics Letters 96 (18): 183102. doi:10.1063/1.3419932.  edit
  6. ^ Lalmi, B.; Oughaddou, H.; Enriquez, H.; Kara, A.; Vizzini, S. B.; Ealet, B. N.; Aufray, B. (2010). "Epitaxial growth of a silicene sheet". Applied Physics Letters 97 (22): 223109. doi:10.1063/1.3524215.  edit
  7. ^ Bernard, R.; Leoni, T.; Wilson, A.; Lelaidier, T.; Sahaf, H.; Moyen, E.; Assaud, L. C.; Santinacci, L.; Leroy, F. D. R.; Cheynis, F.; Ranguis, A.; Jamgotchian, H.; Becker, C.; Borensztein, Y.; Hanbücken, M.; Prévot, G.; Masson, L. (2013). "Growth of Si ultrathin films on silver surfaces: Evidence of an Ag(110) reconstruction induced by Si". Physical Review B 88 (12). doi:10.1103/PhysRevB.88.121411.  edit
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  9. ^ a b c d e f g h i Jose, Deepthi, and Ayan Datta. "Structures and Chemical Properties of Silicene: Unlike Graphene." Accounts of Chemical Research 47.2 (2013): 59312095509002-602. Print.
  10. ^ a b c d e f g Ni, Zeyuan, HongXia Zhong, Xinhe Jiang, Ruge Quhe, Yangyang Wang, Jinbo Yang, Junjie Shi, and Jing Lu. "Tunable Band Gap and Doping Type in Silicene by Surface Adsoprtion: Towards Tunneling Transistors." Academy for interdisciplinary Studies, Peking University, Beijing 100871 P.R. China 1 (2013): 1-30. Print.
  11. ^ Padova, P. D.; Leandri, C.; Vizzini, S.; Quaresima, C.; Perfetti, P.; Olivieri, B.; Oughaddou, H.; Aufray, B.; Le Lay, G. L. (2008). "Burning Match Oxidation Process of Silicon Nanowires Screened at the Atomic Scale". Nano Letters 8 (8): 2299–2304. doi:10.1021/nl800994s. PMID 18624391.  edit
  12. ^ a b Vogt, P.; De Padova, P.; Quaresima, C.; Avila, J.; Frantzeskakis, E.; Asensio, M. C.; Resta, A.; Ealet, B. N. D.; Le Lay, G. (2012). "Silicene: Compelling Experimental Evidence for Graphenelike Two-Dimensional Silicon". Physical Review Letters 108 (15). doi:10.1103/PhysRevLett.108.155501.  edit
  13. ^ Lin, C. L.; Arafune, R.; Kawahara, K.; Tsukahara, N.; Minamitani, E.; Kim, Y.; Takagi, N.; Kawai, M. (2012). "Structure of Silicene Grown on Ag(111)". Applied Physics Express 5 (4): 045802. doi:10.1143/APEX.5.045802.  edit
  14. ^ Feng, B.; Ding, Z.; Meng, S.; Yao, Y.; He, X.; Cheng, P.; Chen, L.; Wu, K. (2012). "Evidence of Silicene in Honeycomb Structures of Silicon on Ag(111)". Nano Letters 12 (7): 3507–3511. doi:10.1021/nl301047g. PMID 22658061.  edit
  15. ^ Guo, Z. X.; Furuya, S.; Iwata, J. I.; Oshiyama, A. (2013). "Absence of Dirac Electrons in Silicene on Ag(111) Surfaces". Journal of the Physical Society of Japan 82 (6): 063714. arXiv:1211.3495. Bibcode:2013JPSJ...82f3714G. doi:10.7566/JPSJ.82.063714.  edit
  16. ^ Arafune, R.; Lin, C. -L.; Nagao, R.; Kawai, M.; Takagi, N. (2013). "Comment on "Evidence for Dirac Fermions in a Honeycomb Lattice Based on Silicon"". Physical Review Letters 110 (22). doi:10.1103/PhysRevLett.110.229701.  edit
  17. ^ Lin, C. L.; Arafune, R.; Kawahara, K.; Kanno, M.; Tsukahara, N.; Minamitani, E.; Kim, Y.; Kawai, M.; Takagi, N. (2013). "Substrate-Induced Symmetry Breaking in Silicene". Physical Review Letters 110 (7). doi:10.1103/PhysRevLett.110.076801.  edit
  18. ^ Gori, P.; Pulci, O.; Ronci, F.; Colonna, S.; Bechstedt, F. (2013). "Origin of Dirac-cone-like features in silicon structures on Ag(111) and Ag(110)". Journal of Applied Physics 114 (11): 113710. doi:10.1063/1.4821339.  edit
  19. ^ Chen, L.; Liu, C. C.; Feng, B.; He, X.; Cheng, P.; Ding, Z.; Meng, S.; Yao, Y.; Wu, K. (2012). "Evidence for Dirac Fermions in a Honeycomb Lattice Based on Silicon". Physical Review Letters 109 (5). arXiv:1204.2642. doi:10.1103/PhysRevLett.109.056804.  edit
  20. ^ Fleurence, A.; Friedlein, R.; Ozaki, T.; Kawai, H.; Wang, Y.; Yamada-Takamura, Y. (2012). "Experimental Evidence for Epitaxial Silicene on Diboride Thin Films". Physical Review Letters 108 (24). doi:10.1103/PhysRevLett.108.245501.  edit
  21. ^ Meng, L.; Wang, Y.; Zhang, L.; Du, S.; Wu, R.; Li, L.; Zhang, Y.; Li, G.; Zhou, H.; Hofer, W. A.; Gao, H. J. (2013). "Buckled Silicene Formation on Ir(111)". Nano Letters 13 (2): 685–690. doi:10.1021/nl304347w. PMID 23330602.  edit
  22. ^ Morishita, T.; Spencer, M. J. S.; Kawamoto, S.; Snook, I. K. (2013). "A New Surface and Structure for Silicene: Polygonal Silicene Formation on the Al(111) Surface". The Journal of Physical Chemistry C 117 (42): 22142. doi:10.1021/jp4080898.  edit
  23. ^ Gao, J.; Zhao, J. (2012). "Initial geometries, interaction mechanism and high stability of silicene on Ag(111) surface". Scientific Reports 2. doi:10.1038/srep00861.  edit
  24. ^ Sugiyama, Y.; Okamoto, H.; Mitsuoka, T.; Morikawa, T.; Nakanishi, K.; Ohta, T.; Nakano, H. (2010). "Synthesis and Optical Properties of Monolayer Organosilicon Nanosheets". Journal of the American Chemical Society 132 (17): 5946. doi:10.1021/ja100919d. PMID 20387885.  edit
  25. ^ Spencer, M. J. S.; Morishita, T.; Mikami, M.; Snook, I. K.; Sugiyama, Y.; Nakano, H. (2011). "The electronic and structural properties of novel organomodified Si nanosheets". Physical Chemistry Chemical Physics 13 (34): 15418. doi:10.1039/c1cp21544b. PMID 21769363.  edit
  26. ^ Spencer, M. J. S.; Morishita, T.; Bassett, M. R. (2013). "Density functional theory calculations of phenol-modified monolayer silicon nanosheets". Micro/Nano Materials, Devices, and Systems. Micro/Nano Materials, Devices, and Systems 8923. pp. 89230D. doi:10.1117/12.2033776.  edit

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