AVS2016 Session 2D-ThA: Surface Chemistry, Functionalization, Bio and Sensor Applications of 2D Materials

Nearly all existing nanoelectronic sensors are based on charge detection, where molecular binding changes the charge density of the sensor and leads to sensing signal. However, there are several fundamental limitations to the charge-detection based electronic sensors. The examples include the ionic screening effect in high ionic strength solution, and the sensitivity-speed tradeoff for vapor phase sensing. In this talk, I will discuss our group’s recent works on a new paradigm of electronic sensing by exploring the heterodyne mixing response between the molecular dipole and a nanoscale transistor. First, we successfully demonstrated that the fundamental ionic screening effect can be mitigated by operating single-walled carbon nanotube field effect transistor as a high-frequency heterodyne biosensor. Electrical detection of streptavidin binding to biotin in 100 mM buffer solution is achieved at a frequency beyond 1 MHz. The results should promise a new biosensing platform for point-of-care detection, where biosensors functioning directly in physiologically relevant condition are desired. Second, we demonstrated the concept of nanoelectronic heterodyne sensor for vapor detection in a graphene device. The dipole detection mechanism is confirmed by a plethora of experiments with vapor molecules of various dipole moments, particularly, with cis- and trans-isomers that have different polarities. Rapid (down to 0.1 s) and sensitive (down to 1 ppb) detection of a wide range of vapor analytes is achieved, representing orders of magnitude improvement over state-of-the-art nanoelectronics sensors. Finally, we demonstrated electrical probing and tuning of the non-covalent physisorption of polar molecules on graphene surface by using graphene nanoelectronic heterodyne sensors. Our results provide insight into small molecule-nanomaterial interaction dynamics and signify the ability to electrically tailor interactions, which can lead to rational designs of complex chemical processes for catalysis and drug discovery.

Although graphene has been widely studied for its 2D properties, its zero band gap nature limits its potential role in semiconducting applications. Molybdenum disulphide (MoS₂) is a semiconductor whose bandgap changes from indirect to direct due to the disappearance of the inversion symmetry when the material is in a monolayer form and in addition this breaking of the symmetry between the K and K' valleys intensifies its use in valleytronic applications. Understanding the correlation between electrical and optical characteristics of MoS₂ is important in order to realize optoelectronic devices based on these materials, including for sensing and biological applications.

In the present work, we investigate and compare the electrical transport characteristics of MoS₂ in monolayer and bilayer forms under the influence of an optical excitation and temperature, through the realization and measurement of MoS₂ interdigitated patterned devices. MoS₂ monolayer and bilayer sheets were synthesized by chemical vapor deposition in an oxygen-free environment on basal plane sapphire substrates. Interdigitated metal contacts were realized using conventional optical lithography with channel lengths ranging from 5 to 10 micrometer. The response of the resulting device was characterized as a function of incident light intensity, wavelength, applied bias, and temperature. We further investigate the impact of these properties on the realization of chemical sensor devices using MoS₂.

Atomically-thin 2D materials such as graphene, boron nitride, or transition metal dichalcogenides can radically alter the chemistry and physics of surfaces they are placed on. Indeed, the appropriate choice of 2D material and subsequent chemical functionalization can dictate all the principal surface forces including van der Waals, acid-base interactions, electric double layers, and even magnetism. For instance, while graphene completely screens the van der Waals forces of the underlying substrate, boron nitride is completely transparent to these forces. A second example is hydrogenation which enables rapid patterning of ferromagnetic domains in graphene. Such control over surface forces should enable us to master technologically critical processes ranging from ice formation to bacterial adhesion to oriented crystal growth. The methods we have developed for chemically functionalizing and patterning graphene will be presented as well as experimental and theoretical work describing how these changes control the various surface forces. Finally, we will discuss a new technique to transfer surface functionalities in toto from one substrate to another.

The unique electronic band structure in single layer WS₂ provides the ability to selectively populate a desired valley by exciting with circularly polarized light. The valley population is reflected through the circular polarization of photoluminescence (PL). We investigate the circularly polarized PL in WS₂ monolayers synthesized on SiO₂/Si substrates using chemical vapor deposition (CVD).^[1] The resulting polarization is strongly dependent on the sample preparation. As-grown CVD WS₂ (still on the growth substrate) exhibits PL emission from the neutral exciton and polarized emission that is unaffected by laser power. Removing WS₂ from the growth substrate and repositioning on the same substrate significantly impacts the optical properties. In transferred films, the excitonic state is optically controlled via high-powered laser exposure such that subsequent PL is from either the charged exciton state or the neutral exciton state, similar to the recently observed behavior in mechanically exfoliated WS₂ flakes.^[2] Additionally, the neutral excitonic emission in transferred CVD films exhibits low polarization whereas the trion polarization can exceed 25% at room temperature, demonstrating the ability to optically control the degree of circularly polarized emission. The removal process may modify the strain, sample-to-substrate distance, and chemical doping in the WS₂ monolayer, and work is underway to determine how these factors influence the valley populations. This work was supported by core programs at NRL and the NRL Nanoscience Institute, and by the Air Force Office of Scientific Research #AOARD 14IOA018-134141.

[1] K. M. McCreary, A. T. Hanbicki, G. G. Jernigan, J. C. Culbertson, B. T. Jonker, Sci. Rep.2016, 6, 19159.

[2] M. Currie, A. T. Hanbicki, G. Kioseoglou, B. T. Jonker, Appl. Phys. Lett.2015, 106, 201907.

Metallic island covered substrates for Surface Enhanced Infrared Absorption (SEIRA) [1] and Surface Enhanced Raman Scattering (SERS) [2] are excellent candidates for enhancement templates in biosensing applications. The specific enhancement behavior of the surfaces is due to their effective optical properties as well as localized and coupled surface plasmon effects. For a detailed understanding of this behavior Au and Ag silver island films on SiO₂/Si with a lateral gradient in size distribution were characterized by IR laser ellipsometry [3] and VIS ellipsometry [4] in combination with numerical and analytical calculations. Along the gradient of the metallic island film, measurements of the IR enhancement of vibrational bands of a Self-Assembling Monolayer adsorbed on the surface as well as a band of the SiO₂ below the island film were performed. The metallic island substrates increase the detection limit and enable new applications, anisotropic substrates allow to separate signals from solvated molecules from adsorbed ones. [2] On the way towards applications as biosensors, the effective and reliable functionalization of these surfaces is an important step. For this we studied a direct electrochemical functionalization of the metal particles from diazonium compounds [5-6] and an indirect route by transferring pre-functionalized graphene sheets to our substrates. [7] For quantitative evaluation the surfaces were characterized in a multi-method approach using UV-VIS ellipsometry, IR ellipsometry, IR microscopy, Raman spectroscopy and IR-AFM.

References

[1] C. Kratz, T. W. H. Oates, K. Hinrichs, Thin Solid Films (2016) in press.

[2] D. Gkogkou, B. Schreiber, T. Shaykhutdinov, H.K. Ly, U. Kuhlmann, U. Gernert, S. Facsko, P. Hildebrandt, N. Esser, K. Hinrichs, I.M. Weidinger, T.W.H. Oates, ACS Sensors, 1 (2016) 318-323.

[3] A. Furchner et al, submitted to Appl. Surface Science.

[4] D. Gkogkou et al, submitted to Appl. Surface Science

[5] P. Kanyong, G. Sun, F. Rösicke, V. Syritski, U. Panne, K. Hinrichs, and J. Rappich; Electrochem. Commun. 51, 103 (2015).

[6] X. Zhang, A. Tretjakov, M. Hovestaedt, G. Sun, V. Syritski, J. Reut, R. Volkmer, K. Hinrichs, and J. Rappich; Acta Biomaterialia 9, 5838 (2013).

[7] F. Rösicke et al, to be submitted.