Magnetism of Topological Boundary States Induced by Boron Substitution in Graphene Nanoribbons

Graphene nanoribbons (GNRs), low-dimensional platforms for carbon-based electronics, show the promising perspective to also incorporate spin polarization in their conjugated electron system. However, magnetism in GNRs is generally associated with localized states around zigzag edges, difficult to fabricate and with high reactivity. Here we demonstrate that magnetism can also be induced away from physical GNR zigzag edges through atomically precise engineering topological defects in its interior. A pair of substitutional boron atoms inserted in the carbon backbone breaks the conjugation of their topological bands and builds two spin-polarized boundary states around them. The spin state was detected in electrical transport measurements through boron-substituted GNRs suspended between the tip and the sample of a scanning tunneling microscope. First-principle simulations find that boron pairs induce a spin 1, which is modified by tuning the spacing between pairs. Our results demonstrate a route to embed spin chains in GNRs, turning them into basic elements of spintronic devices.

Figure 1

(a) Structure of the 2B-7AGNR shown over a color map representing the spin polarization density map, computed by density functional theory simulations ([16]) (green represents the boron moiety). (b) Spin-resolved projected density of states (PDOS) over carbon atoms around the boron dimer. A net spin polarization of one kind confirms the ferromagnetic alignment of the two magnetic moments.

Figure 2

(a) Organic precursors mixed in the experiments. (b) STM constant current topography image of a 2B-7AGNR ($V_b=-300\mathrm{mV}$, $I=30\mathrm{pA}$). The green cross indicates the position from where the GNR is lifted. (right) Constant height current scan ($V_b=2\mathrm{mV}$) using a CO-functionalized tip [38] of the region indicated by the dashed rectangle. (c) Tunneling current $I$ at $V_b=25\mathrm{mV}$ as a function of $z$ for a borylated (red) and a pristine (orange) GNR, for comparison. The grey region indicates where spectra in Fig. 3 were measured. The background shows results of atomistic simulations of a retraction stage shown in Fig. 4, for illustration.

Figure 3

(a) Spectrum over a larger bias interval, taken at $z=2.70\mathrm{nm}$. The dotted grey line is a fitted Frota function [44] with $\mathrm{HWHM}=6\pm 0.4\mathrm{mV}$. The fitting interval is $|V_b|<20\mathrm{mV}$. The inset shows the evolution of the resonance’s HWHM as a function of $z$. (b) Conductance through the GNR as a function of $V_b$ and $z$. The corresponding height interval is the gray region indicated in Fig. 2. A narrow, zero-bias resonance is observed for $2.7\mathrm{nm}<z<2.9\mathrm{nm}$.

Figure 4

(a)–(c) Relaxed structures of three different configurations of a 2B-7AGNR bridging a gold tip and a Au(111) surface (red arrows indicate the position of the B heteroatoms). Constant spin density isosurfaces are shown over the atomic structure ($1.7\times {10}^{-3}\mathrm{e}/{\AA }^3$, spin up in blue and down in red). Insets compare spin PDOS over C atoms within the boxed regions around each boron atom for each bridge geometry (solid lines), with the corresponding one of a free 2B-7AGNR (dashed lines). The GNR zigzag termination on the surface holds a spin-polarized radical state, absent at the contacted end due to the bond formed with the tip’s apex [7]. The equivalent PDOS are provided for a wider energy window in the SM.

Figure 5

(a) (Left) Constant current STM image of a borylated GNR ($V_b=-300\mathrm{mV}$, $I=30\mathrm{pA}$). (Right) Constant height current image of the marked rectangular region ($V_b=2\mathrm{mV}$) using a CO-functionalized tip. (b) DFT simulation of the magnetization of a $(2\mathrm{B}{)}_2$-7AGNR. (c) Cotunneling current $I$ vs $z$ through the ribbon in (a) suspended between tip and sample. (d) Normalized differential conductance of the suspended $(2\mathrm{B}{)}_2$-7AGNR as a function of $V_b$ and $z$ (see SM [16]). Two zero-bias resonances are observed. (e) Representative $dI/dV$ spectra measured at the indicated $z$ positions, with fits (dashed) using Frota functions [44]. (f) DFT relaxed structure of a suspended $(2\mathrm{B}{)}_2$-7AGNR. Constant spin-density isosurfaces are shown over the atomic structure; red arrows indicate the position of the B atoms. The inset shows the indicated region of the suspended GNR from a different angle.