Monday, June 12, 2017

A graphene-based digital camera

An imaging chip that replaces silicon with a novel pixel material is sensitive to more than just visible light.

Steven K. Blau

At the heart of a smartphone camera is CMOS circuitry that registers the electrons produced when visible light strikes a silicon wafer. Other semiconductors coupled to CMOS circuits could enable cameras to image in the UV, IR, and terahertz bands; such detectors could see applications for night vision, food inspection, environmental monitoring, and more. Now a research team led by Gerasimos Konstantatos and Frank Koppens of the Institute of Photonic Sciences (ICFO) in Barcelona, Spain, has taken the first steps toward that possible future: It has coupled a graphene–quantum dot photodetector to a CMOS circuit to create an imaging chip sensitive to wavelengths ranging from 300 nm to 1850 nm. (The group’s earlier related work was discussed in Physics Today, July 2012, page 15.)

The researchers’ device, whose surface is roughly 15 mm × 15 mm, has almost 120 000 active pixels, plus a row of insensitive “blind” pixels. The left side of the figure shows a side-view schematic of a single pixel. A graphene layer lies atop the CMOS; the photosensitive material, lead sulfide quantum dots, is deposited on the graphene. When light hits a PbS quantum dot it creates electron–hole (e–h) pairs. The holes enter the graphene layer, where they flow due to a voltage applied across each pixel. The device measures light intensity by comparing the current in an active pixel with that in the blind pixels. The right side of the figure shows the result: a view of a pear and apple illuminated with IR light. (S. Goossens et al., Nat. Photonics11, 366, 2017.)

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