The 83rd JSAP Autumn Meeting 2022 Highlighted Presentations
October 24, 2022

High-spatial-resolution infrared spectroscopy with graphene light emitter

Developing Inexpensive, High-Resolution Infrared Analysis
Technology with Graphene, a Miraculous Material, Opens
New Possibilities for Applications in Chemistry, Physics,and Bio-Science
Faculty of Science and Technology, Keio University (1), Kagami Memorial Research Institute for Materials
Science and Technology (2), Center for Spintronics Research Network, Keio University (3)Kenta Nakagawa (1),(2), Yui Shimura (1), Yusuke Fukazawa (1), Ryosuke Nishizaki (1), Shinchiro Matano(1), Shuma Oya (1), Hideyuki Maki (1),(3)summary• Originally developed multilayer graphene light-emitter chip enables infrared analysis witha high spatial resolution of 1 μm, which is beyond the diffraction limit.
• With a light source as small as 1/1,000,000 in area infrared analysis similar to that ofconventional FT-IR infrared thermal light sources is now possible.
• The technique is inexpensive and has high spatial resolution, and it is expected to findapplications in chemistry, physics, and bio-science.

A research group led by Prof. Hideyuki Maki, Dr. Kenta Nakagawa and Mr. Yui Shimura of theDepartment of Applied Physics & Physico-lnfomatics, Faculty of Science and Technology, Keio
University has developed a multilayer graphene light-emitter chip using multilayer graphene as alight-emitting element, and they developed a new infrared spectroscopy technique. This technique
has achieved an extremely high spatial resolution of 1 μm, exceeding the theoretical resolution
limit (i.e., diffraction limit) that has been a barrier in conventional infrared analysis techniques suchas Fourier-transform infrared spectroscopy (FT-IR). The technique is inexpensive and easy to use
and is therefore expected to find applications in chemistry, physics, bio-science.

detailInnovation in Infrared Analysis with Graphene, a Miraculous Material
Graphene, a new material derived from graphite (the material used to make pencil lead), is
an extremely thin, light, and tough sheet-like material consisting of a single twodimensional layer of carbon atoms. Owing to its unique physical properties, it is being used
in applications such as biosensors and power generation. Graphene has also beenreported to have unique optical properties. Prof. Hideyuki Maki’s research group at theDepartment of Applied Physics & Physico-lnfomatics, Faculty of Science and Technology,
Keio University, has developed an innovative infrared analysis technology by using aproprietary multilayer graphene light-emitter chip as a light-emitting device (Figure).
Infrared analysis is an analytical technique for structural analysis and quantification inwhich an object is irradiated with infrared light, following which the transmitted andreflected light are examined using a spectrometer. However, a major problem withconventional infrared analysis techniques such as FT-IR is their resolution. Opticalsystems such as microscopes are limited in what they can measure owing to the diffraction
limit, that is, the theoretical limit of spatial resolution caused by the diffraction of light*. For
example, a general microscope cannot accurately observe objects that are smaller than
the wavelength of light owing to the diffraction limit.
The same applies to infrared analysis. Halogen lamps and ceramic light sources used aslight sources in conventional infrared analysis techniques such as FT-IR are infrared lightsources with macroscopic wavelengths of the order of millimeters. Therefore, thediffraction limit is as low as 10 μm. In other words, performing high-resolution imaging andmicroanalysis is difficult with conventional infrared analysis based on visible light.
Dr. Kenta Nakagawa of the research group noted that the diffraction limit can be overcome
by directly using the near-field light emitted by a multilayer graphene light-emitting device
as the light source.
Inexpensive, high-resolution near-field light produced by graphene
Near-field light has a special property in that it does not propagate through space when it
is irradiated through a hole with a size smaller than its wavelength or onto a nanosizedmaterial. Unlike scattered light, which generally travels through space, near-field light ischaracterized by the absence of diffraction. Therefore, it can be used as a light source toobserve objects smaller than the wavelength of light.
To directly use the near-field light emitted by a light source, the object to be observed mustbe brought close to the light source. However, the halogen lamps and ceramic lightsources that are currently used widely in infrared analysis are like miniature light bulbs. Inboth, the light source is enclosed in a three-dimensional object such as glass. Therefore,

the sample cannot be brought close to the light-emitting part. By contrast, the graphenelight-emitter chip developed by Prof. Maki’s research group in the Department of Physics
and Information Engineering, Keio University (published in Nature Communications in2018), has a planar structure produced by depositing graphene, the light-emitting part, ona silicon substrate and attaching electrodes on top. Graphene blackbody radiation sources
with a minimum size of 500 nm2 can also be produced. Dr. Nakagawa noted that thesefeatures can be exploited to bring the sample close to the light-emitting part and use nearfield light.
Attempts have been made to utilize near field light to observe nanoscale objects beyond
the diffraction limit. For example, the scanning near-field optical microscope (SNOM) has
been attracting attention as it achieves high spatial resolution in the infrared region.
However, SNOMs are very large and require expensive infrared wavelength tunable
lasers. Further, the laser is extremely difficult to handle and can only be operated byexperts. By contrast, infrared analysis with multilayer graphene light-emitting elements
uses the near-field light generated directly on the graphene surface for imaging. Further,Dr. Nakagawa noted that it is inexpensive and does not require complex technology but
can nonetheless achieve high-resolution infrared analysis.
The multilayer graphene light emitter can be directly modulated thousands of times fasterthan a conventional FT-IR infrared thermal light source (capable of blinking 100,000 times
per second). This feature enables infrared analysis similar to that of conventional FT-IRinfrared thermal light sources, despite the fact that the light source is very small(approximately area of 1/1,000,000). The spatial resolution is 1 μm, which is well beyond
the diffraction limit.
The new FT-IR infrared light source enables the same level of imaging and micro/trace
analysis as visible light, but at a much lower cost and with greater simplicity in the infrared
region. Dr. Nakagawa noted that this feature enables applications in fields such as
chemistry, physics, bio-science where infrared analysis has not been possible thus far.
!annotation"#Diffraction of Light: Light has the properties of a wave; however, when it strikes an obstacle in its direction of travel, it
goes around behind it. This phenomenon is called diffraction of light.
$figures(a) Schematic of multilayer graphene light emitter infrared analysis. The near-field light is localized just above thegraphene. (b) IR transmittance spectra of polymethylpentene as measured by conventional FT-IR and multilayer
graphene source IR analysis. (c) 2D IR transmittance image of Ni pattern obtained with an optical microscope and an IRsystem using multilayer graphene light emitter IR analysis. The spatial resolution is high and approaches that of visible
light.