Direct writing of heterostructures in single atomically precise graphene nanoribbons

Precision control of interfacial structures and electronic properties is the key to the realization of functional heterostructures. Here, utilizing the scanning tunneling microscope (STM) both as a manipulation and characterization tool, we demonstrate the fabrication of a heterostructure in a single atomically precise graphene nanoribbon (GNR) and report its electronic properties. The heterostructure is made of a seven-carbon-wide armchair GNR and a lower band gap intermediate ribbon synthesized bottom-up from a molecular precursor on an Au substrate. The short GNR segments are directly written in the ribbon with a STM tip to form atomic precision intraribbon heterostructures. Based on STM studies combined with density functional theory calculations, we show that the heterostructure has a type-I band alignment, with manifestations of quantum confinement and orbital hybridization. Our finding demonstrates a feasible strategy to create a double-barrier quantum dot structure with atomic precision for functionalities, such as negative differential resistance devices in GNR-based nanoelectronics.

Figure 1

(a) Temperature-dependent growth process of bottom-up synthesis of the 7-aGNRs using DBBA molecules as precursors. After the formation of polyanthrylene, subsequent annealing at a temperature of 600 K will create an intermediate state with one-side cyclodehydrogenation while annealing to 670 K will fully convert the polymer chains to the 7-aGNRs. (b) Large-area STM image after partial graphitization at 600 K (sample voltage $V_{\mathrm{s}}=-2.0\mathrm{V}$, tunneling current $I_{\mathrm{t}}=100\mathrm{pA})$. Residual polymer and intermediate segments are marked with red and white dashed boxes, respectively. (c) Upper panel: High-resolution STM image showing the atomic structure of an intermediate $\left(V_{\mathrm{s}}=-0.5\mathrm{V},I_{\mathrm{t}}=100\mathrm{pA}\right)$. Lower panel: Structural model of the intermediate.

Figure 2

(a) Schematic of the direct writing of HSs with an STM tip. (b) Left: STM images of a short intermediate segment with four units embedded in a 7-aGNR before and after a pulse treatment $(V_{\mathrm{s}}=-2.0\mathrm{V}$, pulse time $t=30\mathrm{ms})$ at the black-cross-marked site $\left(V_{\mathrm{s}}=-2.0\mathrm{V},I_{\mathrm{t}}=100\mathrm{pA}\right)$. Right: Schematic of the STM tip–induced reaction marked by the dashed arrow. (c) Sequential direct writing of GNR segments in the intermediate to form HSs at designed molecular sites marked by crosses. Conversion of two neighboring intermediate units in (i) to a GNR segment in (ii), and sequentially of another unit of the intermediate in (ii) to a GNR segment in (iii) (set point: $V_{\mathrm{s}}=-2.0\mathrm{V},I_{\mathrm{t}}=20\mathrm{pA})$. The direct writing is enabled by STM pulses at $V_{\mathrm{s}}=-5.0\mathrm{V},I_{\mathrm{t}}=20\mathrm{pA}$, and time $t=30\mathrm{ms}$. The corresponding structural models are shown in lower panels. (d) Representative $I\text{−}t$ curves at different biases. For all $I\text{−}t$ curves the tip was stabilized with a set point of $V_{\mathrm{s}}=-2.0\mathrm{V}$ and $I_{\mathrm{t}}=100\mathrm{pA}$ before switching off the feedback loop. The arrows mark the abrupt changes in the $I\text{−}t$ curves and the corresponding biases.

Figure 3

(a) $\mathrm{d}I/\mathrm{d}V$ spectra acquired along the black dashed line across the interface between the 7-aGNR and an intermediate shown in the upper panel of (b). A gray curve is acquired on the bare Au surface as reference, showing a typical Au surface state at about −0.45 V. Blue and red curves are from the 7-aGNR and the intermediate segment, respectively. The HOMO and LUMO peaks for each segment are marked. (b) Upper panel: STM of a GNR/intermediate HS, showing the polymer side edge. Lower panel: Color-coded $\mathrm{d}I/\mathrm{d}V$ map plotted with the measured curves in (a). (c) Upper panel: Structural model showing the polymer side edge of the GNR/intermediate junction. Lower panel: Calculated LDOS map. The pink dashed line marks the junction location, and the white lines mark the HOMO and LUMO levels for corresponding segments.

Figure 4

$\mathrm{d}I/\mathrm{d}V$ curves taken on the intermediate (Int.) segments with different lengths as shown in Fig. 2, in comparison with a curve from the 7-aGNR. The band gaps are labeled to show a confinement effect.

Figure 5

(a) STM image showing 7-aGNR/intermediate HS $\left(V_{\mathrm{s}}=-2.0\mathrm{V},I_{\mathrm{t}}=40\mathrm{pA}\right)$. (b) $\mathrm{d}I/\mathrm{d}V$ curves acquired at cross-marked sites in (a): curve A (gray) is measured on the bare Au surface, curve B (blue) on the fully converted 7-aGNR edge, curve C (red) on the polymer side of the intermediate, and curve D (green) on the graphitized side of the intermediate. (c) Structural model of the intermediate. (d) Calculated LDOSs correspondingly obtained from the dashed boxes in (c).

Figure 6

(a,b) STM topographic images (upper panels) and simultaneously acquired $\mathrm{d}I/\mathrm{d}V$ maps (lower panels) at $-0.6$ and $+1.5\mathrm{V}$, respectively $\left(I_{\mathrm{t}}=100\mathrm{pA}\right)$. (c,d) Simulated STM images (upper panels) at marked energies and charge density distributions (lower panels) at the energies of ${\mathrm{HOMO}}_{\mathrm{i}}$ and ${\mathrm{LUMO}}_{\mathrm{i}}$ of the intermediate, respectively. Subhexagon resolved structures are marked with red and black arrows. ${\mathrm{HOMO}}_{\mathrm{i}}$ and ${\mathrm{LUMO}}_{\mathrm{i}}$ are both extended in the intermediate in (c,d).