Extended Abstract – Summer 2015 REU-MRSEC

Investigation of interlayer interactions on MoSe2-WSe2 heterostructures in different substrates

Andre M. S. Magnavita1, Demi Ajayi² and James Hone²*.1Federal University of Sao Carlos, ²Columbia University.

1andre_mengel@hotmail.com, ²oaa2114@columbia.edu, or jh2228@columbia.edu *.

Abstract: Heterostructures connected by van der Waals forces, constructed from two-dimensional (2D) semiconductors, such as transition metal dichalcogenides (TMDCs) are very important for the development of new concepts within the atomic physics and emerging technologies of inter-conversion between light and electricity. Monolayers of TMDCs exhibit an emerging photoluminescence (PL), which opens up a great range of opportunities for optoelectronic applications using these materials. When stacking monolayers of these materials to form heterostructure (HS) devices, it can give rise to bound electron-hole pairs across the interface, i.e., inter-layer or charge transfer (CT) excitons, due to the poorly screened Coulomb potential in the interface. In order to investigate the interlayer interactions between MoSe2 and WSe2 and possible interferences of the substrate in the measurements, HS devices were fabricated by the ultra-thin method, by producing monolayers of these materials from mechanical exfoliation and stacking the layers by using the “pick-up” method, yielding high quality devices. We performed photoluminescence spectroscopy to characterize the monolayers and to study the devices made in the substrates of SiO2/Si and Sapphire, with or without the presence of a bottom hBN piece. AFM were performed in order to investigate the structural quality of the stacks, enabling explain variations in the PL signal.

IntroductionThe discovery of new properties on two-dimensional (2D) materials, high

mobility of electrons or strong photoluminescent response when their thickness are reduced from bulk to a few layers or even monolayers has been surprising scientists, confirming physics concepts and opening many possibilities for technological innovations and creating advantages of improved efficiencies, possibility of reduced dimensions and low cost. The different possibilities to build heterostructures (HS), connecting different 2D materials, like graphene and transition metal dichalcogenides (TMDCs), by van der Waals forces, has an important role in the creation of new technologies of light-electricity inter-conversion, such as photo-detectors, solar cells and light emitting diodes1. In this work, we are going to focus on TMDCs properties, more specifically on the investigation of interlayer interactions in MoSe2-WSe2

heterostructures, testing light emission response of the devices on different substrates.

The TMDCs has the formula MX2, where M are transition metals like Molybdenum (Mo) or Tungsten (W), and X can be the chalcogens Sulfur (S) or Selenium (Se). These materials are semiconductors, generally have an indirect bandgap and exhibit photoluminescence (PL) response after exposure to a monochromatic laser when they are composed of multilayers. In spite having similar crystalline structures, their physical properties may vary significantly and therefore, it is possible to stack different TMDCs monolayers to form different HSs2. Recently were found that TMDCs

Extended Abstract – Summer 2015 REU-MRSEC

exhibit an indirect to direct bandgap transition when their thickness are reduced to a single monolayer. This effect leads to a strong emerging PL response from the monolayers.

When two TMDC monolayers are stacked to form HTs devices, they typically present a type II band alignment, which works similarly as that of a p-n junction in a photovoltaic cell or photo-detector1. Because in 2D TMDCs van der Waals bound heterobilayers the Coulomb binding energy is much stronger than in conventional semiconductors, it can lead to interlayer excitonic states, rising a bound electron and electron-hole pair across the interface, with the electron and the hole localized in different layers. This bound state is called inter-layer or charge transfer (CT) exciton and it can lead to a photocurrent generation that can be documented through PL measurements at room temperatures, Second Harmonic Generation and Pump Probe tests.

Methods and MaterialsMonolayer fabrication and characterization. Monolayers of MoSe2 and WSe2, and few layers of hexagonal Boron Nitride (hB) were prepared by scotch tapes with tiny crystals of these materials and they were used to mechanically exfoliate onto 285nm SiO2 on heavily doped Si wafers. Before the exfoliation, the wafers were cleaned by plasma etching, exposed for different times to compare quality and size of the flakes acquired. The monolayers were identified with an optical microscope through color contrast and confirmed by PL spectroscopy measurements, using a green laser, wavelength of 532nm.

Devices fabrication. After characterization of the flakes, monolayers of MoSe2 and WSe2, as well as few-layers flakes of hBN, were chosen by size and shape in order to get a better fit for the device and enable different PL measurements in the final heterostructure. To build the HS, the “Pick-up” method was used. First PDMS lenses were produced on top of glass slides, following a circular shape with a certain diameter. A good size is important to make the pick-up and transfer processes to be very smooth, avoiding many problems with wrinkles or cracks in the samples. The PDMS lenses were placed in the Transfer Station and the wafers were fixed to metal plates by applying vacuum. After aligning the center of the PDMS with hBN flake, they were put in contact and the metal plate was heated until 50 C. After reaching the set temperature, we waited some minutes before starting to cool down the sample and as well as slowly raising the samples. The hBN flake on PDMS coated with PPC is used to pick-up the MoSe2 monolayers, using the same procedure with a more careful approach to align the flakes before pick-up. Repeated with WSe2. After the full stack is made, the PPC was melt at 120 C directed on the surface of SiO2/Si, or on top of bottom hBN on Sapphire or SiO2/O2. The samples were cleaned with acetone for 3 hours and later annealed in order to remove polymer residues.

Devices Characterization and investigation of CT excitons. The devices were scanned with the Atomic Force Microscope (AFM) to check the integrity of the structure of stacks and amount of polymer residue left in the samples. The devices were submitted to a PL mapping scan to investigate the features of the heterostructures, monolayer peaks and CT exciton peak, by using a green laser with wavelength of 532nm.

Extended Abstract – Summer 2015 REU-MRSEC

Results and DiscussionDuring the program, three heterostructure devices were built, each one with a

different combination of substrate with or without hBN in order to investigate which fabricate way is better to document the CT exciton peak and interferences that the substrate can cause in the PL signal.

The sapphire substrate is very transparent and looks gray under the microscope, making the typical technique of using optical contrast to identify monolayers difficult. Stack A can be seen in the Figure 1a below. The SiO2/Si is a conducting substrate that provides a very good optical color contrast of the monolayers and stack region, as shown in 1b, but when the stack is placed on top of a thick hBN flake, the contrast decrease drastically (1c).

Figure 1. Electron microscope image showing the stack region: (a) On top of thick hBN flake in sapphire substrate. (b) Directly on SiO2/Si substrate. (c) On top of thick flake hBN in SiO2/Si substrate.

The AFM image of the sample A (2a) shows a small amount of polymer residue left in the sample and good structural integrity of the stack without apparent big cracks. The AFM image of the sample B (2b) has a very poor contrast due to the small difference in thickness between the substrate, the stack and monolayers regions, but it is possible to see a good amount of polymer residues in the top hBN layer. Polymer residues are also present in small dots in the top hBN on AFM image of the sample C (2c), but the stack structure don’t present cracks or wrinkles, which enable better PL measurements.

Extended Abstract – Summer 2015 REU-MRSEC

Figure 2. AFM image of: (a) Stack region on top of hBN in sapphire substrate showing almost clean surface of stack. (b) Stack and monolayer regions of the heterostructure direct on SiO2/Si substrate, with no presence of cracks on the sample. (c) Stack region on top of hBN in SiO2/Si2 substrate with presence of small polymer residue dots on the surface of stack.

In order to investigate the CT exciton peak and monolayer peaks in the stack, we conducted PL map scans on the substrate, monolayer and stack regions in the heterostructure made. The PL spectra responses for all stacks show a much higher photoluminescent response peak for the WSe2 monolayer at approximately 1.65 eV with an asymmetric feature already expected, previously seen during the monolayer characterization process, as can be seen in the PL responses below (3a-c). The MoSe2 monolayer peak response is a little hidden by the large WSe2 monolayer signal, but it can be noticed at approximately 1.59 eV. Unfortunately, in this preliminary investigation and data analysis, it was not possible to see the CT exciton peak around 1.35 eV.

Figure 3. Photoluminescent Response from a green laser with wavelength of 532nm on the stack region: (a) On top of thick hBN flake in sapphire substrate. (b) Directly on

Extended Abstract – Summer 2015 REU-MRSEC

SiO2/Si substrate. (c) On top of thick flake hBN in SiO2/Si substrate. All the PL responses have a very strong PL for WSe2 monolayer and an asymmetric feature.

ConclusionThis research was focused in the fabrication of MoSe2-WSe2 heterostructure

devices made with monolayers of these materials, encapsulated or not with hBN in different substrates, in order to see possible interferences of the substrate in the ability to document the charge transfer exciton in the stack region. In this preliminary data analysis, the CT exciton peak was not found in any of the devices built, but for a more consistent analysis of the devices and investigation of the interlayer interactions in these heterostructures, it is necessary to do a further investigation by using Pump Probe spectroscopy and Second Harmonic Generation (SHG) for a temporal resolution of the PL signal and also determine the crystal orientation of the monolayers and align the stacks.

References1. ZHU, X.; MONAHAN, N. R.; GONG, Z.; ZHU, H.; WILLIAMS, K. W.;

NELSON, C. A. J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b03141.2. Rivera, P. et al. Observation of long-lived interlayer excitons in monolayer

MoSe2–WSe2 heterostructures. Nat. Commun. 6:6242. doi: 10.1038/ncomms7242 (2015).

AcknowledgmentsFunding for this scientific research was provided by the Institute of International Education – Brazil Scientific Mobility Program Academic Training. I acknowledge partial support from the Center for Precision Assembly of Superstratic and Superatomic Solids: an NSF MRSEC under award number DMR-1420634. I would like to thanks James Hone, Demi Ajayi and Ghidewon Arefe for all the support and mentoring provide during all the research. I also would like to thanks Jessica Balbinot, Jenny Ardelean and Dario Vasquez.

In the next years, I would like to go to Graduate School and then start to work in an industry as a Chemical Engineer.