Two-dimensional materials are nearly invisible and offer enormous potential

Scientists at the Center for Advanced Graphene, Nanomaterials and Nanotechnology Research are studying remarkable new materials that can be used to develop photonic devices for future generations.

By Heitor Shimizu, in New York  |  Agência FAPESP – The Center for Advanced Graphene, Nanomaterials and Nanotechnology Research (MackGraphe) at Mackenzie Presbyterian University are exploring remarkable new materials that can be used to develop such things as photonic devices for future generations, in a variety of fields and applications.  

Funded by FAPESP, MackGraphe, which began activities in 2013, “is dedicated to the science and industrial application of graphene and other two-dimensional materials, and has three primary areas of interest: photonics, energy and composite materials,” said Christiano José Santiago de Matos, a professor at Mackenzie and a researcher at MackGraphe, in a presentation given at FAPESP Week New York, jointly organized by the City University of New York (CUNY) and the Wilson Center November 26-28, 2018 at the Graduate Center of CUNY. 
 
Two-dimensional (or 2D) materials are made of single layers of atoms. Graphene was the first of these materials to be isolated in 2004, an achievement that resulted in the Nobel Prize in Physics 2010 awarded to Andre Geim and Konstantin Novoselov.

Graphene corresponds to a graphite crystal that is the thickness of one carbon atom. It is hundreds of times stronger than steel and conducts heat and electricity far better than copper, which makes it a material that offers enormous potential to the semiconductor and electronics industries.  

“It’s a superlative material. It’s a million times thinner than a human hair and extremely light – just 3 grams of graphene can cover a football field. It’s practically invisible to the naked eye, absorbing only 2.3% of visible light,” Matos said.    

“Graphene has the highest known thermal conductivity, 10 times higher than copper, and it is an excellent conductor of electricity. In addition to that, it is a very strong material, 200 times stronger than steel,” he said.    

But graphene is not the only 2D material under analysis at MackGraphe. Another material offering enormous potential is black phosphorous, an accordion-structured crystal composed of two-dimensional layers of phosphorous stacked in a monoatomic layer.

“Since graphene’s isolation in 2004, atomically-thin crystals are finding an increasing number of applications in areas as diverse as electronics, biomedical sensing and materials engineering,” said Matos.  

The diminutive thickness of 2D material causes them to interact with light differently. “The interaction between light and 2D materials has shown itself to be very strong, particularly in the case of semiconductors, not only enabling the observation of peculiar effects but also suggesting a variety of photonic applications,” Matos said.    

“There are several reasons why 2D materials are so good for optics and photonics, such as their interaction with light (despite being practically transparent) and the possibility of adjusting their properties according to the number of layers. They are also materials that are easily stacked, forming more complex and functional structures,” he said.    

Characterizing new materials

During the presentation, Matos also talked about the use of spectroscopy to identify and study the particular characteristics of the new 2D materials. The technique used is based on a phenomenon observed in experiments conducted in 1928 by Indian Chandrasekhara Venkata Raman. It uses a source of laser, which when reaching an object, is scattered by the object, generating light that is either the same or a different frequency than the incident light.   

In the first case, the scattering is called elastic or Rayleigh scattering. Most important, however, is the second case: the inelastic effect, also known as Raman scattering, which allows us to obtain important information about the chemical composition and structure of the material being studied from this frequency difference. This generates a signature of the chemical composition and structure of each irradiated material.     

In addition to studying the structural properties of the edges of black phosphorous crystals using Raman spectroscopy, finding peculiar characteristics, Matos and his colleagues explored a more sensitive version of the technique for the demonstration of a chemical sensor.    

“It is what is known as Surface Enhanced Raman Spectroscopy, or SERS, a high sensitivity photonics technique that provides chemical and structural information about molecules in low concentrations and in seconds,” said Matos.     

In September, Matos and his colleagues at MackGraphe published an article in the journal Optics Express that demonstrated the chemical sensor, utilizing a glass microcapillary fiber as an optofluidic platform – combining the advantages of microfluidics and optics.  

To enhance the Raman scattering, the internal orifice of the fiber was coated with a graphene oxide and gold nanorod nanocomposite. “The fiber allows for the detection of the molecules at acquisition times as low as 0.05 seconds, indicating the potential for use in real-time sensing, requiring just over 100 nanoliters of sample,” Matos said.   

Matos and his colleagues point out that the detection of chemical elements at ultralow concentrations “is of utmost importance for sensing applications in several areas, especially in environmental monitoring, life sciences and homeland security.” Among these applications is the early diagnosis of diseases and the detection of explosives, chemical warfare agents and pollutants.    

Coherence in disorder

Another speaker during the session on photonics at FAPESP Week was Azriel Genack, a professor in the Physics Department of Queens College and the Graduate Center, within the CUNY system, who talked about the challenges in trying to find coherence in transport in disordered media.

For the past decade, Genack has studied the classical wave propagation in the presence of disorder. One of the objectives is to find coherence in the apparently chaotic transmission in various means, such as microwaves (10 GHz to 20 GHz) or laser (630 nanometers). 

“Classical waves are the means by which we probe our environment and communicate with one another. As a result of wave-particle duality, studies of classical waves also serve as models of electronic transport, involving quantum mechanical waves, in a solid state,” Genack said.  

“One goal of our studies at Queens College of optical and microwave radiation propagation is to provide a universal description of wave scattering in random systems. We have been able to demonstrate the relationship between the statistics of fluctuations of intensity and total transmission, non-local intensity correlation, and average transport in space, time and frequency,” he added.   

The researcher said that this has led to the development of essential models of electronic transport in mesoscopic systems, which are systems in which the phase coherence of the wave is preserved throughout the sample. 

For more information about FAPESP Week New York, visit: www.fapesp.br/week2018/newyork.