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Similarities and Differences with Graphene

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.^[1] While both form hexagonal structures, only the graphene is completely flat, while the silicene structure forms a buckled hexagonal shape.^[1] 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.^[1] 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.^[1]

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^[1]. 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.^[1]

Properties

The buckling of the hexagonal structure of silicene is caused by pseudo-Jahn-Teller distortion (PJT)^[1]. This is caused by strong vibronic coupling of unoccupied molecular orbitalsunoccupied 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^[1]. 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^[1].

Tunable Band Gap

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 ^[2]. 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)^[2].

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^[2]. 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^[2]. Through platinum (Pt) doping, i-type silicene is possible^[2]. 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^[2]. 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^[2].

References

^ ^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.
^ ^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.

[scp-1] ^ ^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.

[TBG-2] ^ ^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.

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