Modeling nanoscale gas sensors under realistic conditions: Computational screening of metal-doped carbon nanotubes

We use computational screening to systematically investigate the use of transition-metal-doped carbon nanotubes for chemical-gas sensing. For a set of relevant target molecules (CO, ${\text{NH}}_3$, and ${\text{H}}_2\text{S}$) and the main components of air (${\text{N}}_2$, ${\text{O}}_2$, and ${\text{H}}_2\text{O}$), we calculate the binding energy and change in conductance upon adsorption on a metal atom occupying a vacancy of a (6,6) carbon nanotube. Based on these descriptors, we identify the most promising dopant candidates for detection of a given target molecule. From the fractional coverage of the metal sites in thermal equilibrium with air, we estimate the change in the nanotube resistance per doping site as a function of the target molecule concentration assuming charge transport in the diffusive regime. Our analysis points to Ni-doped nanotubes as candidates for CO sensors working under typical atmospheric conditions.

Figure 1

(Color online) Calculated geometry reconstructions for the (a) monovacancy, (b) the divacancy I, and (c) the divacancy II in a (6,6) carbon nanotube. Atoms on the far side of the nanotube are not shown for clarity. The atoms involved in the reconstructed bonds are shown in red.

Figure 2

(Color online) Structural schematics and binding energies (namely, formation energies with the opposite sign) for a $3d$ transition metal occupied monovacancy (blue), divacancy I (green), or divacancy II (red) in a (6,6) carbon nanotube. Binding energies of a carbon atom (two in the case of divacancies) in the empty vacancies are indicated by lines with the same color code.

Figure 3

(Color online) Adsorption energy, $E_{\text{ads}}$, in eV for ${\text{N}}_2$, ${\text{O}}_2$, ${\text{H}}_2\text{O}$, CO, ${\text{NH}}_3$, and ${\text{H}}_2\text{S}$ on $3d$ transition metals occupying a Monovacancy (top), Divacancy I (middle), and Divacancy II (bottom) in a (6,6) carbon nanotube.

Figure 4

(Color online) Fractional coverage, $\Theta$ in thermal equilibrium of Ni in a (a) monovacancy, (b) divacancy I, and (c) divacancy II, per dopant site as a function of CO concentration in a background of air at room temperature and 1 bar of pressure. $C_0=0.1\text{ppm}$.

Figure 5

(Color online) Calculated change in conductance $\Delta G$, in units of $G_0=2e^2/h$, for ${\text{N}}_2$, ${\text{O}}_2$, ${\text{H}}_2\text{O}$, CO, ${\text{NH}}_3$, and ${\text{H}}_2\text{S}$ on $3d$ transition metals occupying a monovacancy (top), divacancy I (middle), and divacancy II (bottom) in a (6,6) carbon nanotube.

Figure 6

Change in resistance, $\Delta R$, per Ni-occupied dopant site as a function of CO concentration in a background of air at room temperature and 1 bar of pressure. $C_0=0.1\text{ppm}$. Note the change from linear to log scale on the $y$ axis at $\Delta R=10\Omega$.

Figure 7

(Color online) [(a)–(f)] Structures of a (6,6) CNT with a transition-metal atom (pink atom) adsorbed in a monovacancy and in the two divacancies considered in this work. Monovacancy is showed in (a) (side view) and (b) (top view). Divacancy I is showed in (c) (side view) and (d) (top view). Divacancy II is showed in (e) (side view) and (f) (top view). In all the figures the carbon atoms bonded to the TM are highlighted in red (longer bonds) and blue (shorter bonds). (g) Difference in bond length between the two inequivalent TM-C bonds formed around the TM-doped monovacancies (blue circles), divacancies I (green squares), and divacancies II (red triangles).