Bromine-adsorption-induced change in the electronic and magnetic properties of nanographite network systems

Nanographite having open edges has a nonbonding $\pi$-electron state of edge origin (edge state), which causes unconventional electronic and magnetic features. Bromine-adsorption effect on the electronic and magnetic properties of nanoporus carbon consisting of a flexible three-dimensional random nanographite network system is investigated by using activated carbon fibers (ACFs) as host material. The irreversible adsorption of bromine into ACFs at room temperature gives compositions up to $\mathrm{Br}∕\mathrm{C}=0.43$. The interaction between nanographite and the adsorbed bromine is classified into three groups; charge transfer, covalent bonding, and physisorption interactions, where the latter two are the majorities. Although the charge transfer rate from carbon to bromine is considerably small (0.0004 per C atom at maximum), in comparison with that in bulk graphite-bromine intercalation compounds, the downshift of the Fermi energy results in the large reduction of the localized spin concentration of edge state and the orbital diamagnetism. This proves the important role of the edge state in the charge transfer process. The electron spin resonance results demonstrate the contribution of the orbital character of bromine to the edge state in nanographite through orbital mixing. The physisorbed bromine accommodated into the nanopores induces the dielectric and structural effects on the nanographite. The former causes the modification of the carrier conduction process due to the charging effect, while the latter results in the magnetic switching phenomenon induced by the effective pressure of physisorbed bromine species to the edge-state spins.

Figure 1

The bromine adsorption isotherm for ACFs at $298\mathrm{K}$. $p_0$ are the saturated vapor pressure $\left(33.3\mathrm{kPa}\right)$ at $298\mathrm{K}$. Closed circles and open circles denote the adsorption process for the virgin sample, and the subsequent desorption process, respectively. The arrows indicate the sequence of the process.

Figure 2

TG traces for nonadsorbed ACFs (closed circles), $\mathrm{Br}∕\mathrm{C}=0.27$ (closed triangles), and $\mathrm{Br}∕\mathrm{C}=0.067$ (open circles). Scan rate is $100°\mathrm{C}∕\min$. (a) $w$ vs $T$ plot. (b) $dw∕dT$ vs $T$ plot. Weight change $w$ in the vertical axis is normalized with the initial weight at room temperature $w_{\mathrm{R}\mathrm{T}}=1$.

Figure 3

Temperature dependence of the mass fragment peak intensity integrated in the mass number range between 79 and 82, which are assigned to bromine atom and HBr species, for $\mathrm{Br}∕\mathrm{C}=0.27$ with a heating rate of $10°\mathrm{C}∕\min$.

Figure 4

CT rate $f_{\mathrm{c}}$ per carbon atom for Br-ACFs as a function of bromine content.

Figure 5

(a) The bromine content dependence of the electrical conductivity for Br-ACFs at room temperature. (b) The time dependence of the conductivity in the bromine desorption process for the Br-ACFs $(\mathrm{Br}∕\mathrm{C}=0.43)$ at room temperature. The conductivity is normalized with respect to that for the nonadsorbed ACFs ${\sigma }_0$.

Figure 6

The temperature dependence of the conductivity $\sigma$ for (a) the nonadsorbed ACFs obeying $\ln \sigma$ vs $T^{-1∕2}$ law (solid line) and (b) Br-ACFs with $\mathrm{Br}∕\mathrm{C}=0.27$ obeying the $\ln \sigma$ vs $T^{-1∕4}$ law (solid line).

Figure 7

${\chi }_{\mathrm{c}}^{-1}$ vs $T$ plots for Br-ACFs $(0⩽\mathrm{Br}∕\mathrm{C}⩽0.27)$. ●, ◻, ▴, ▿, ◆, 엯, ∎, and ▵ denote the data for the nonadsorbed ACFs and Br-ACFs with $\mathrm{Br}∕\mathrm{C}=0.038$, 0.067, 0.083, 0.10, 0.14, 0.16, and 0.27, respectively. The inset shows schematically three divided temperature regions (L, M, H), which are obtained based on the difference in the susceptibility behavior.

Figure 8

(a) ${\chi }_{\mathrm{c}}T$ vs $T$ plots for Br-ACFs $(0⩽\mathrm{Br}∕\mathrm{C}⩽0.27)$. ●, ◻, ▴, ▿, ◆, 엯, ∎, and ▵ denote the data for the nonadsorbed ACFs and Br-ACFs with $\mathrm{Br}∕\mathrm{C}=0.038$, 0.067, 0.083, 0.10, 0.14, 0.16, and 0.27, respectively. (b) The temperature dependence of the mass density of bulk bromine (Refs. 40, 41).

Figure 9

The bromine content dependence of the localized spin concentration $N_{\mathrm{s}}$. The solid lines are the guide for the eyes.

Figure 10

The bromine content dependence of the orbital susceptibility ${\chi }_{\mathrm{orb}}$ in the field perpendicular to the graphitic plane. The value of ${\chi }_{\mathrm{orb}}$ is obtained by multiplying the observed temperature independent term ${\chi }_0$ by 3 after the subtraction of the Pascal diamagnetic susceptibility. The value of ${\chi }_0$ has ambiguity of less than 10% due to the contribution of the Pauli paramagnetism. The solid line is the guide for the eyes.

Figure 11

The bromine content dependence of the ESR linewidth $\Delta H_{\mathrm{pp}}$ (a) and the $g$-value (b) at room temperature. The $g$-value is denoted as the difference from that of the free electron spin $(\Delta g=g-2.0023)$.

Figure 12

The schematical band structure of actual nanographite in ACFs. $\pi$ and ${\pi }^{*}$ are the graphitic linear bonding and antibonding bands with the density of states $D\left(E\right)=8\mid E\mid ∕3\pi a^{2}c{\gamma }_0^2$, respectively. The Gaussian function with half width $W$ and the peak height $D\left(0\right)$ in the density of states is assumed for the edge-state band. The vertical dashed and solid lines denote the position of the Fermi energy $E_{\mathrm{F}}$, and the degenerate point, respectively. $E_{\mathrm{F}}$ is measured by the shift from the degenerate point. Dotted lines denote the characteristic points of the electronic structure.

Figure 13

The CT rate $f_{\mathrm{c}}$ dependence of (a) the localized spin density $n_{\mathrm{s}}$ per gram of carbon in the intermediate temperature range M, (b) the Fermi energy, and (c) the orbital susceptibility ${\chi }_{\mathrm{orb}}$ per gram of carbon. The experimental data (full circles) in (a) and (c) are obtained after Figs. 4, 9, 10. The horizontal dashed line in (a) is the CT-independent contribution of the localized spins. The solid curve in (a) is the fitting curve to the model shown in Fig. 12 (see text). The calculated $n_{\mathrm{s}}$ is obtained on the assumption that the nanographite mass density is the same as that of bulk graphite $2.25\mathrm{g}{\mathrm{cm}}^{-3}$ (Ref. 31). The solid curve in (b) is obtained from the fitting (solid curve) in (a). The $E_{\mathrm{F}}$ vs $f_{\mathrm{c}}$ plot in the case of the absence of the edge state is denoted as the dashed line in (b). The solid curve in (c) is the ${\chi }_{\mathrm{orb}}$ at $300\mathrm{K}$ calculated with the results in (b) based on McClure’s theory (Ref. 39). The values for ${\chi }_{\mathrm{orb}}$ in (c) are corrected with respect to the systematic experimental error (see text).

Figure 14

The schematic description of the inhomogeneous charge transfer between nanographite and bromine. BL and IL denote the bounding layers faced to bromine, and interior layers of nanographite, respectively. The edge-state spins and bromine are schematically shown as the straight arrows and circles, respectively. The edge-state spins disappear on the bounding layers due to the charge transfer.