The lack of a sizeable band gap has so far prevented graphene from building effective electronic and optoelectronic devices despite its numerous exceptional properties. Intensive theoretical research reveals that a band gap larger than 1 eV can only be achieved in sub-3 nm wide graphene nanoribbons (GNRs), but real fabrication of such ultranarrow GNRs still remains a critical challenge. Herein, we demonstrate an approach for the synthesis of ultranarrow and photoluminescent semiconducting GNRs by longitudinally unzipping single-walled carbon nanotubes. Atomic force microscopy reveals the unzipping process, and the resulting 2.2 nm wide GNRs are found to emit strong and sharp photoluminescence at ∼685 nm, demonstrating a very desirable semiconducting nature. This band gap of 1.8 eV is further confirmed by follow-up photoconductivity measurements, where a considerable photocurrent is generated, as the excitation wavelength becomes shorter than 700 nm. More importantly, our fabricated GNR field-effect transistors (FETs), by employing the hexagonal boron nitride-encapsulated heterostructure to achieve edge-bonded contacts, demonstrate a high current on/off ratio beyond 105 and carrier mobility of 840 cm2/V s, approaching the theoretical scattering limit in semiconducting GNRs at room temperature. Especially, highly aligned GNR bundles with lengths up to a millimeter are also achieved by prepatterning a template, and the fabricated GNR bundle FETs show a high on/off ratio reaching 105, well-defined saturation currents, and strong light-emitting properties. Therefore, GNRs produced by this method open a door for promising applications in graphene-based electronics and optoelectronics.

Surface plasmon polaritons (SPPs) provide subwavelength electric field confinement ranging from microwave to the visible. Several approaches have been explored to manipulate SPPs with a typical modulation capability of only a few percent. Here, active control of SPPs using monolithically fabricated Schottky diodes has been first designed and realized, achieving a continuous and significant modulation of transmission, reflection, and absorption. The SPPs propagating on a metal line are attenuated by using split ring resonators (SRRs) with an In-Ga-Zn-O Schottky diode bridging each SRR split gap. The resistance of the diodes can be tuned over a range of a few orders of magnitude with bias voltage, which continuously transforms each SRR to a metallic quasi-loop with subdued resonance. A remarkable modulation of 40% and 19% was demonstrated for the transmission and absorption, respectively, which to the best of our knowledge has not been achieved before. Graphic entry for the Table of Contents (TOC)

A novel band-stop filter with single-loop split ring resonators (SRRs) is proposed for spoof surface plasmon polaritons (SPPs) at millimeter wave frequencies, achieving a miniaturized size of 0.052λ0×0.278λ0 at its resonant frequency. The SRRs provide both a low-pass response as the rectangular corrugations used in the conventional SPPs and an additional band-stop response induced by the resonance of SRRs. To verify this design, a back-to-back device with two co-planar waveguides as the input and output feeding was fabricated and characterized, the measured S-parameters of which agree well with the simulation. The measured stop band is centered at 49 GHz with a -10-dB bandwidth of 4.1 GHz and a high Q-factor of 93, in which the maximum attenuation is 31 dB. The filter has a low insertion loss of less than 2.8 dB in the pass band. Such approaches may find many applications to achieve compact millimeter wave circuits.

Surface plasmon polaritons (SPPs) are propagating electromagnetic surface waves with local electric field enhancement and nondiffraction limit at optical frequencies. At terahertz (THz) frequencies, a metal line with periodic grooves can mimic the optical SPPs with the same high cut-off response, which is referred to as designer SPPs. Here, by replacing metal grooves with graphene sheets, a novel active metal–graphene hybrid SPP device achieves significant phase modulation. Theoretically, the dispersion curves of THz SPPs are determined by the dimensions and periodicity of the grooves. Changing the chemical potential of graphene sweeps the effective groove depth, which correspondingly shifts the SPP cut-off frequency and modulates the slow-wave phase. A prototype device is fabricated and characterized under varying bias applied for graphene. The experiment demonstrates that the cut-off frequency red shifts from 200 to 177 GHz, and the phase variation is as large as 112° at 195 GHz under a low bias from −0.5 to 0.5 V. Simultaneously, the SPP transmittance is modulated by a factor of more than 3 dB from 140 to 177 GHz due to the graphene absorption. The proposed structure reveals a novel approach to study the nonreciprocal and topological SPPs with active modulation in the THz range.

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