Irene Calizo

A theoretical study of gas adsorption on silicene nanoribbons and its application in a highly sensitive molecule sensor

Inspired by recent successes in the development of two-dimensional based gas sensors capable of single gas molecule detection, we investigate the adsorption of gas molecules (N2, NO, NO2, NH3, CO, CO2, CH4, SO2, and H2S) on silicene nanoribbons (SiNRs) using density functional theory (DFT) and nonequilibrium Greens function (NEGF) methods. The most stable adsorption configurations, adsorption sites, adsorption energies, charge transfer, quantum conductance modulation, and electronic properties of gas molecules on SiNRs are studied. Our results indicate that NO, NO2, and SO2 are chemisorbed on SiNRs via strong covalent bonds, suggesting its potential application for disposable gas sensors. In addition, CO, NH3, and H2S are chemisorbed on SiNRs with moderate adsorption energy, alluding to its suitability as a highly sensitive gas sensor. The quantum conductance is detectably modulated by chemisorption of gas molecules which can be attributed to the charge transfer from the gas molecule to the SiNR. Other studied gases are physisorbed on SiNRs via van der Waals interactions. It is also found that the adsorption energies are enhanced by doping SiNRs with either B or N atoms. Our results suggest that SiNRs show promise in gas molecule sensing applications.Inspired by the recent successes in the development of two-dimensional based gas sensors capable of single gas molecule detection, we investigate the adsorption of gas molecules such as N2, NO, NO2, NH3, CO, CO2, CH4, SO2, and H2S on silicene nanoribbons using density functional theory and nonequilibrium Greens function methods. The most stable adsorption configurations, adsorption sites, adsorption energies, charge transfer, quantum conductance modulation, and electronic band structures of all studied gas molecules on SiNRs are studied. Our results indicate that NO, NO2, and SO2 are chemisorbed on SiNRs via strong covalent bonds, suggesting its potential application for disposable gas sensors. In addition, CO and NH3 are chemisorbed on SiNRs with moderate adsorption energy, alluding to its suitability as a highly sensitive gas sensor. The quantum conductance is detectably modulated by chemisorption of gas molecules which can be attributed to the charge transfer from the gas molecule to the SiNR. Other studied gases are physisorbed on SiNRs via van der Waals interactions. It is also found that the adsorption energies are enhanced by doping SiNRs with either B or N atom. Our results suggest that SiNRs show promise in gas molecule sensing applications.

Band gap tuning of armchair silicene nanoribbons using periodic hexagonal holes

The popularity of graphene owing to its unique and exotic properties has triggered a great deal of interest in other two-dimensional nanomaterials. Among them silicene shows considerable promise for electronic devices with a carrier mobility comparable to graphene, flexible buckled structure, and expected compatibility with silicon electronics. Using first-principle calculations based on density functional theory, the electronic properties of armchair silicene nanoribbons perforated with periodic nanoholes (ASiNRPNHs) are investigated. Two different configurations of mono-hydrogenated (:H) and di-hydrogenated (:2H) silicene edges are considered. Pristine armchair silicene nanoribbons (ASiNRs) can be categorized into three branches with width W = 3P − 1, 3P, and 3P + 1, P is an integer. The order of their energy gaps change from “EG (3P − 1) < EG (3P) < EG (3P + 1)” for W-ASiNRs:H to “EG (3P + 1) < EG (3P − 1) < EG (3P)” for W-ASiNRs:2H. We found the band gaps of W-ASiNRs:H and (W + 2)-ASiNRs:2H are slight...

Edge functionalization and doping effects on the stability, electronic and magnetic properties of silicene nanoribbons

Through density functional theory calculations the impact of edge functionalization on the structural stabilities, electronic and magnetic properties of silicene nanoribbons (SiNRs) are investigated. –H, –F, –Cl, –Br, and –I edge functionalization of armchair, zigzag, Klein, reconstructed Klein, reconstructed pentagon–heptagon edge types, and their combinations are examined. It is found for the first time that trifluorinated Klein edge SiNR is the most stable edge structure among all edge structures of SiNRs. Furthermore, the stability of trihydrogenated Klein edge SiNR, which is periodically replaced by a dihydrogenated zigzag edge, approaches that of dihydrogenated armchair edge SiNR as the most stable hydrogenated SiNR. It is revealed that asymmetry in edge functionalization or combining different edge types can transform symmetric edge functionalized zigzag SiNRs from antiferromagnetic semiconductors to various magnetic states, such as bipolar spin gapless semiconductors (SGS), ferromagnetic metals and semiconductors, and antiferromagnetic metals. Furthermore, the effects of N or B doping on the stability, electronic and magnetic properties of hydrogenated and fluorinated SiNRs are studied. It is discovered that the mono-fluorinated armchair SiNR shows SGS behavior with 100% spin polarized currents around the Fermi level, when the Si edge atom is substituted by an N or B atom. The remarkable SGS and half-metal characters, and ferromagnetic metals are also observed in N- or B-doped asymmetric edge functionalized zigzag SiNRs, fully functionalized Klein edge SiNRs, and combinations of zigzag SiNRs with reconstructed Klein edge SiNRs. These results encourage further experimental investigations in the development of SiNR-based nanoelectronics with spin tuning.

DFT study of adsorption behavior of NO, CO, NO2, and NH3 molecules on graphene-like BC3: A search for highly sensitive molecular sensor

The adsorption behavior of toxic gas molecules (NO, CO, NO2, and NH3) on graphene-like BC3 are investigated using first-principle density functional theory (DFT). The most stable adsorption configurations, adsorption energies,binding distances,charge transfers,electronic band structures,and the conductance modulations are calculated to deeply understand the impacts of the molecules above on the electronic and transport properties of the BC3 monolayer. The graphene-like BC3 monolayer is a semiconductor with a band gap of 0.733 eV. The semi-metal graphene has a low sensitivity to the abovementioned molecules. However, it is discovered that all the above gas molecules are chemically adsorbed on the BC3 sheet with the adsorption energies less than -1 eV. The NO2 gas molecule is totally dissociated into NO and O species through the adsorption process, while the other gas molecules retain their molecular forms. The amounts of charge transfer upon adsorption of CO and NH3 gas molecules on BC3 are found to be small. Hence, the band gap changes in BC3 as a result of interactions with CO and NH3 are only 4.63% and 16.7%, indicating that the BC3-based sensor has a low and moderate sensitivity to CO and NH3, respectively. Contrariwise, upon adsorption of NO or NO2 on BC3, a significant charge is transferred from the molecules to the BC3 sheet, causing a semiconductor-metal transition. It is found that the BC3-based sensor has high potential for NO detection due to the significant conductance changes, moderate adsorption energy, and short recovery time. More excitingly, the BC3 is a likely catalyst for dissociation of the NO2 gas molecule. Our findings divulge promising potential of the graphene-like BC3 as a highly sensitive molecular sensor for NO and NH3 detection and a catalyst for NO2 dissociation

Edge functionalized germanene nanoribbons: impact on electronic and magnetic properties

Germanene exhibits extremely high mobility, massless fermion behavior, and strong spin–orbit coupling drawing tremendous interest for high performance devices. It has a buckled two-dimensional structure, but not the intrinsic energy band gap and structural stability required for logic and switching devices. Application of a perpendicular electric field, surface adsorption, confinement of an armchair nanoribbon structure and edge functionalization are methods used to open a band gap. Edge functionalization of armchair germanene nanoribbons (AGeNRs) has the potential to achieve a range of band gaps. The edge atoms of AGeNRs are passivated with hydrogen (–H and –2H) or halogen (–F, –Cl, –OH, –2F, –2Cl) atoms. Using density functional theory calculations, we found that edge functionalized AGeNRs had band gaps as small as 0.012 eV when functionalized by –2H and as high as 0.84 eV with –2F. Formation energy studies revealed that AGeNRs produced a more stable structure under fluorine functionalization. Simulation results suggest that the electronic structure of germanene is similar to graphene and silicene. A spin-polarized density functional theory (DFT) study of electronic and magnetic properties of pristine, chemically functionalized and doped AGeNRs and zigzag nanoribbons (ZGeNRs) was performed. Formation energy studies revealed that the Ge atoms at the edge of the ribbon prefer to be replaced by impurity atoms. Doping can change the semiconducting behaviour of AGeNRs to metal behaviour due to the half-filled band making it useful for negative differential resistance (NDR) devices. In the case of ZGeNRs, single N or B doping transformed them from anti-ferromagnetic (AFM) semiconducting to ferromagnetic (FM) semiconducting or half-metal. These magnetic and electronic properties make edge functionalized doped AGeNRs and ZGeNRs promising for use in field effect transistors (FETs) and spintronics. Finally, energy band gap tuning of AGeNRs and ZGeNRs using edge functionalization may open a new route to integrate germanene in logic and high performance switching devices.

Emergence of strong ferromagnetism in silicene nanoflakes via patterned hydrogenation and its potential application in spintronics

Considerably different properties emerge in nanomaterials as a result of quantum confinement and edge effects. In this study, the electronic and magnetic properties of quasi zero dimensional silicene nanoflakes (SiNFs) are investigated using first principles calculations. Whilst the zigzag edged hexagonal SiNFs exhibit nonmagnetic semiconducting character, the zigzag edged triangular SiNFs are magnetic semiconductors. One effective method of harnessing the properties of silicene is hydrogenation owing to its reversibility and controllability. From bare SiNFs to half hydrogenated and then to fully hydrogenated, a triangular SiNF experiences a change from ferrimagnetic to very strong ferromagnetic, and then to non-magnetic. Nonetheless, a hexagonal SiNF undergoes a transfer from nonmagnetic to very strong ferromagnetic, then to nonmagnetic. The half hydrogenated SiNFs produce a large spin moment that is directly proportional to the square of the flakes size. It has been revealed that the strong induced spin magnetizations align parallel and demonstrates a collective character by large range ferromagnetic exchange coupling, giving rise to its potential use in spintronic circuit devices. Spin switch models are offered as an example of one of the potential applications of SiNFs in tuning the transport properties by controlling the hydrogen coverage.

Structural Stability of Functionalized Silicene Nanoribbons with Normal, Reconstructed, and Hybrid Edges

Silicene, a novel graphene-like material, has attracted a significant attention because of its potential applications for nanoelectronics. In this paper, we have theoretically investigated the structural stability of edge-hydrogenated and edge-fluorinated silicene nanoribbons SiNRs via first-principles calculations. Various edge forms of SiNRs including armchair edge, zigzag edge, Klein edge, reconstructed Klein edge, reconstructed pentagon-heptagon edge, and hybrid edges have been considered. It has been found that fully fluorinated Klein edge SiNRs, in which each edge Si atom is terminated by three fluorine atoms, are the most stable structure. We also discovered that a hybrid edge structure of trihydrogenated Klein edge and dihydrogenated zigzag edge can increase the nanoribbon’s stability up to that of dihydrogenated armchair edge SiNR, which is known as the most stable edge-hydrogenated structure. With the attractive properties of silicene for practical applications, the obtained results will advance experimental investigations toward the development of silicene based devices.

Bandgap changes in armchair silicene nanoribbons perforated with periodic nanoholes

In this study, electronic properties of armchair silicene nanoribbon perforated with periodic nanoholes are studied by first-principle calculations based on density functional theory. It has been demonstrated that pristine armchair silicene nanoribbons can be categorized into three branches with width W= 3P-1, 3P, and 3P+1, where P is a positive integer. Their energy gaps decrease as a function of increasing nanoribbon width and satisfy EG (3P-1) <; EG (3P) <; EG (3P+1). The bandgaps of armchair silicene nanoribbons perforated with periodic nanoholes show an oscillatory behavior and become smaller or larger than that of pristine armchair silicene nanoribbon depending on the nanoribbons width and repeat periodicity of nanoholes. Results indicate that the different nanoribbons width and edge shape between two adjacent nanoholes and between nanoholes and nanoribbons in a small repeat periodicity will result in different quantum confinement effects. It has been also shown that the bandgap of armchair silicene nanoribbon are closely dependent on the position of nanoholes relative to edges of the nanoribbons.