Respectively they are a zero band-gap conductor and wide band-gap insulator.
“When we put them together, you form a band gap mis-match that creates a potential barrier that stops electrons,”said Michigan professor Yoke Khin Yap.
The team took exfoliated graphene and grew BN tubes on (see diagram) it with chemical vapour deposition. 60nm diameter hetrojunctions formed at the joins.
Tungsten scanning tunnelling microscope (STM) probes were used to contact the graphene and the tube.
When the probe was 1.23µm up the tube from the join there was no conduction (+/-30V bias).
At 620nm away from the join, a symmetrically-conducting junction formed – with a curve much like a diode conduction curve, but bidirectional, which was conducting significantly (0.5µA) with 30V bias in either duirection.
Attempts to turn the junction into a transistor – the under-lying oxidised silicon wafer (500nm oxide) was used as a back-gate – were unsuccessful, even at +/-100V. The team proposed that highly-conductive lower layers of the multi-layer graphene shielded the junction, and that a top-gated structure might work.
Simulation by density functional theory (DFT) suggests that mismatch of the density of states (DOS) is responsible for the voltage-dependant switching behaviour, according to ‘Switching behaviors of graphene boron nitride nanotube heterojunctions‘, a paper covering the work in Nature Scientific Reports.
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