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低维材料表面的有毒气体吸附一直是分子传感器领域的研究热点. 最近的新型正交相二维BN单层半导体材料引起了学者的广泛关注. 基于第一性原理计算, 本文研究了有毒气体分子(CO, H2S, NH3, NO, NO2和SO2)在二维正交相BN单层表面的吸附特性. 吸附能计算表明, 正交相BN材料对所有气体的吸附均是能量有利的放热过程.NO2和NH3为化学吸附, 其他体系均为物理吸附, 且NO吸附后体系具有自旋极化的电子能带结构. 态密度计算结果显示, NO2和SO2的吸附对费米能级附近的电子占据有显著影响.NO2吸附后, 体系的功函数和电子结构特性均明显改变, 表明BN单层半导体材料对NO2分子具有良好的吸附选择性和探测灵敏度. 采用第一性原理结合非平衡格林函数方法计算了纳米电子器件的输运性能. 在1 V偏压下, NO2分子的吸附可使体系电流增大到6500 nA, 显著增强了BN单层半导体器件的电输运性能, 因此二维BN单层对NO2气体吸附具有优异的探测敏感度. 此外, 轴向应力能够有效调控体系的吸附性能. 研究表明, 正交相BN单层材料对有毒气体具有优异的吸附性能, 在新型有毒气体传感探测器件领域有较大的应用潜力.The adsorption properties of toxic gases on the surface of low-dimensional nanomaterials are a research hot topic and key issue for developing semiconductor sensors to detect toxic gas molecules. Recently, a novel orthorhombic BN monolayer has attracted extensive attention from researchers. Using first principles calculations, we investigate the adsorption properties of typical toxic gas molecules, such as CO, H2S, NH3, NO, NO2, and SO2 molecules, on the surface of two-dimensional (2D) orthorhombic BN monolayer adsorption. The calculated adsorption energy show that the adsorptions of the above six molecules on the surface of BN monolayer are energy-favorable exothermic processes. It is found that NO2 and NH3 molecules are of chemical adsorption, while other systems are of physical adsorption, and NO adsorbing system exhibits a spin-polarized electronic band structure. The calculated density of states reveals that the adsorption of NO molecule and SO2 molecule have significant influences on the electronic structure near the Fermi level. Moreover, the adsorption of the NO2 molecule on the substrate exhibits remarkable variation of the work function, suggesting that the o-BN monolayer possesses excellent selectivity and sensitivity to NO2 molecule. In addition, we use first principles combined with non-equilibrium Green’s function to simulate the electrical transport properties of monolayered o-BN semiconductor based nanodevice with adsorption of typical toxic gas molecules. The I-Vb curve shows that the current through the nanodevice is 6500 nA for the NO2 molecule adsorbing system under 1 V bias voltage. The calculation results reveal that the adsorption of NO2 molecule on the o-BN monolayer can significantly enhance its electrical transport performance, and the o-BN monolayer possesses excellent sensitivity and selectivity to the NO2 gas molecule. The work function and the charge transfer can be effectively manipulated by tensile strain, indicating its potential application in anisotropic electronics. Our results indicate that the o-BN monolayer has excellent adsorption performance to toxic gases, showing its practical application in capturing toxic gas molecules as a gas sensor in future.
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Keywords:
- adsorption /
- first-principles /
- orthorhombic BN /
- electrical transport /
- gas sensor
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图 1 正交相BN单层原子结构球棍模型的顶视图和侧视图. 绿球和银球分别表示B原子和N原子, 对于气体分子吸附问题, 分子分别放置在3种不同的初始位置(T1, T2, H1), 图中T1和T2分别表示N原子和B原子的顶位, H1表示在六元环中心的空位
Fig. 1. Top view and side views of optimized structure of the pristine BN monolayer. Green and silver balls indicating B and N atoms, respectively. Three different absorption sites are considered, namely, T1, T2, H1, and the gas molecules are initially at these sites. T1 and T2 represent the top sites above the N and B atom, respectively, and H1 represents the hollow site.
图 2 纳米电子器件模型示意图, 它包含左电极、右电极和中心散射区. 左右电极与中心区域无缝连接, 中心区域分子的吸附会对器件的电输运性能产生影响
Fig. 2. Schematic demonstration of the nanodevice consisting of left-, right-electrode and the central region. The left and right electrodes are seamlessly connected with the central region, and the molecule adsorption at the central region will have impacts on the electrical performance of device.
图 3 理想o-BN单层材料的电子结构 (a) 能带结构; (b) 态密度; (c)平均静电势分布
Fig. 3. Electronic structure of the pristine o-BN monolayer: (a) Band structure; (b) the density of states; (c) the averaged potential of the pristine BN monolayer.
图 4 气体在单层o-BN上稳定吸附构象的俯视图和侧视图 (a) CO; (b) H2S; (c) NH3; (d) NO; (e) NO2; (f) SO2; 绿色、银色、棕色、红色、白色、黄色球分别表示B, N, C, O, H和S原子
Fig. 4. Top and side views of the most stable adsorption sites for (a) CO, (b) H2S, (c) NH3, (d) NO, (e) NO2 and (f) SO2 molecules adsorbed on the monolayered BN. The green, silver, brown, red, white, yellow balls indicate B, N, C, O, H and S atoms, respectively.
图 5 o-BN吸附气体后的能带图 (a) CO; (b) H2S; (c) NH3; (d) NO; (e) NO2; (f) SO2, (d)中红色线和蓝色线分别表示自旋向上和向下子能带
Fig. 5. Electronic band structures for (a) CO, (b) H2S, (c) NH3, (d) NO, (e) NO2 and (f) SO2 on o-BN. The red and blue line indicate spin up and down, respectively.
图 6 o-BN单层吸附气体分子的差分电荷密度的俯视图和侧视图 (a) CO; (b) H2S; (c) NH3; (d) NO; (e) NO2; (f) SO2; 其中等值线为0.0005 e·Bohr–3, 黄色和青色分别表示电子聚集和电子耗尽的程度
Fig. 6. Charge density difference contours for (a) CO, (b) H2S, (c) NH3, (d) NO, (e) NO2, and (f) SO2 adsorbed on the o-BN monolayer (0.0003 e·Bohr–3 is set for isosurface value). Yellow and cyan represent electron accumulation and electron depletion, respectively.
图 7 o-BN吸附气体后的态密度分布 (a) CO; (b) H2S; (c) NH3; (d) NO; (e) NO2; (f) SO2
Fig. 7. Calculated densities of states (DOS) for the o-BN monolayer with (a) CO, (b) H2S, (c) NH3, (d) NO, (e) NO2 and (f) SO2 molecules adsorption, respectively.
图 8 吸附不同气体分子后o-BN体系的平均静电势分布 (a) CO; (b) H2S; (c) NH3; (d) NO; (e) NO2; (f) SO2
Fig. 8. Average potential of the o-BN monolayer with (a) CO, (b) H2S, (c) NH3, (d) NO, (e) NO2, and (f) SO2 molecules adsorption, respectively.
图 9 o-BN单层材料吸附气体前后的功函数分布
Fig. 9. Calculated work function of the o-BN monolayer with adsorption of different molecules.
图 10 气体分子吸附前后o-BN单层器件的I-$ {V}_{{\mathrm{b}}} $曲线, 插图展示了NO2吸附的o-BN单层器件完整的I-$ {V}_{{\mathrm{b}}} $结果
Fig. 10. Electrical performance of I-$ {V}_{{\mathrm{b}}} $ curve of pristine o-BN monolayer device and gas molecule adsorbed o-BN monolayer devices. The inset figure exhibits the I-Vb curve of the NO2 adsorbed o-BN monolayer device.
图 11 o-BN单层对NO2气体吸附性能随单轴应力的变化 (a) 吸附距离; (b) 吸附能; (c) 功函数; (d) 电荷转移
Fig. 11. Adsorption performance of BN-NO2 system as a function of uniaxial strain along x and y axis, respectively: (a) Adsorption distance; (b) adsorption energy; (c) work function; (d) electron transfer based on Bader analysis.
表 1 吸附能$ {E}_{{\mathrm{a}}{\mathrm{d}}} $ (eV)、气体与衬底之间的距离d (Å)和电荷转移Q (e), 正值表示气体分子从o-BN衬底获得电子
Table 1. Adsorption energy $ {E}_{{\mathrm{a}}{\mathrm{d}}} $ (eV), distances between gas and the substrate d (Å) and the charge transfer Q (e). The positive value means the gas molecule obtains electron from the BN substrate.
$ {E}_{{\mathrm{a}}{\mathrm{d}}} $/eV d/Å Q/e CO –0.151 2.999 0.032 H2S –0.186 2.488 0.014 NH3 –0.222 1.762 –0.102 NO –0.258 2.284 0.133 NO2 –0.893 1.679 0.780 SO2 –0.380 2.922 0.165 
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表 2 气体在300—600 K之间吸附后的恢复时间τ(s)
Table 2. Recovery time τ(s) after gas adsorption ranging from 300 to 600 K.
Gas
moleculeTemperature/K 300 400 500 600 CO 3.44×10–11 7.99×10–12 3.33×10–12 1.85×10–12 H2S 1.33×10–10 2.20×10–11 7.49×10–12 3.65×10–12 NH3 5.36×10–10 6.26×10–11 1.73×10–11 7.32×10–12 NO 2.16×10–9 1.78×10–10 3.98×10–11 1.47×10–11 NO2 100.13 0.02 1.00×10–4 3.16×10–6 SO2 2.42×10–7 6.13×10–9 6.76×10–10 1.55×10–10
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表 3 气体分子吸附在o-BN单层后的功函数$ \varPhi $(eV)、功函数差$ {{\Delta }}\varPhi $(eV)和灵敏度S(%)
Table 3. Work function $ \varPhi $ (eV), work function difference $ {{\Delta }}\varPhi $ (eV) and sensitivity S (%) of the gas molecules adsorbed on the o-BN monolayer.
Adsorbed gas $ \varPhi $/eV $ {{\Delta }}\varPhi $/eV S/% CO 4.141 0.032 0.78 H2S 4.465 0.356 8.66 NH3 4.203 0.094 2.29 NO 4.103 –0.006 0.15 NO2 5.366 1.257 30.59 SO2 4.599 0.450 10.95
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