Research Article www.acsami.org
Highly Luminescent WS2 Quantum Dots/ZnO Heterojunctions for Light Emitting Devices Arup Ghorai,† Sayan Bayan,‡ Narendar Gogurla,‡ Anupam Midya,*,† and Samit K. Ray*,‡ †
School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur 721302, India Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
‡
S Supporting Information *
ABSTRACT: Sonication induced vertical fragmentation of two-dimensional (2D) WS2 nanosheets into highly luminescent, monodispered, zero-dimensional (0D) quantum dots (QDs) is reported. The formation of 0D structures from 2D sheets and their surface/microstructure characterization are revealed from their microscopic and spectroscopic investigations. Size dependent optical properties of WS2 nanostructures have been explored by UV−vis absorption and photoluminescence spectroscopy. Interestingly, it is observed that, below a critical dimension (∼2 nm), comparable to the Bohr exciton radius, the tiny nanocrystals exhibit strong emission. Finally, the electroluminescence characteristics are demonstrated for the first time, by forming a heterojunction of stabilizer free WS2 QDs and ZnO thin films. The signature of white light emission in the light emitting device is attributed to the adequate intermixing of emission characteristics of WS2 QDs and ZnO. The observation of white electroluminescence may pave the way to fabricate prototype futuristic efficient light emitting devices. KEYWORDS: tungsten sulfide WS2, nanocrystals, transition metal dichalcogenides (TMDC), electroluminescence, quantum confinement, quantum dots
■
INTRODUCTION Low-dimensional transition metal dichalcogenides (TMDCs) consisting triatomic building blocks (X-M-X, M = Mo, W and X = S, Se) have emerged as a new class of semiconducting material having exotic properties. In contrast to other two-dimensional (2D) materials, TMDCs such as WS2, MoSe2, and MoS2 are distinguished by their size and thickness dependent direct band gap energy, attractive for optoelectroic devices. More interestingly, WS2 and MoS2 nanocrystals are environmentally benign and optically stable compared to widely studied semiconducting nanocrystals such as CdS, CdSe, PbS, etc. Size tunable direct band gap energies of 1−3 eV1of TMDCs have been utilized for optoelectronic devices, such as light emitting diodes (LEDs),2 photovoltaic devices,3 photocatalysts,4 photodetectors,5,6 etc. Different top-down and bottom-up techniques have been employed to obtain semiconducting low-dimensional WS2. Mimicking graphene exfoliation by mechanical methods, TMDC can be exfoliated but with poor control of size and yield. Chemical exfoliation using n-butyllithium can be employed for the lithium intercalation between TMDC layers, which reacts with water, yielding metallic TMDCs without any emissive characteristics. Compared to MoS2, the synthesis of WS2 nanosheets by lithium ion exfoliation is quite difficult due to the resistance of WS2 to lithium ion intercalation.7,8 In addition, very often, surfactant is required to stabilize the product.9 Hence, there is an urgent need of devising a method to produce © 2016 American Chemical Society
semiconducting WS2 nanocrystals in a controlled way to explore the physical and chemical properties of WS2 for its applications in devices. A few studies on electroluminescence devices have been reported using nanostructured MoS2, such as a heterojunction composed of a p-type MoS2−MoO3 film and an n-type 4H-SiC to fabricate LEDs exhibiting multiwavelength emission.10 However, the study on the light emission characteristics of WS2 QDs is rare. Exfoliation of bulk WS2 powder to few-layer WS2 nanosheets using lithium halide (LiX, X = Cl, Br, I) as a lithiating agent and the fabrication of heterojunction photodiodes have been recently reported by our group.11 In the present study, we report the synthesis of monodispersed WS2 QDs (down to ∼2 nm size) in a simple sonication induced fragmentation process from WS2 nanosheets to study their tunable absorption and emission behavior. Finally, white light emission characteristics of highly luminescent WS2 QDs based heterojunctions with thin film ZnO nanoparticles are demonstrated for the first time to test the efficacy of WS2 nanocrystals for optoelectronics device applications. Received: October 10, 2016 Accepted: December 13, 2016 Published: December 13, 2016 558
DOI: 10.1021/acsami.6b12859 ACS Appl. Mater. Interfaces 2017, 9, 558−565
Research Article
ACS Applied Materials & Interfaces
Figure 1. Typical atomic force micrographs of as-synthesized WS2 for different centrifugation speeds (a) 4000, (b) 8000, (c) 14 000 rpm and their corresponding size distribution histograms are displayed in the right side of the figure (d−f), respectively, showing the thickness distribution of WS2 nanocrystals.
■
Finally, cleaned ITO coated glass substrates were dip-coated for 30 min to get thin films of ZnO nanoparticles. The ZnO film was annealed at 90 °C at ambient atmosphere for overnight to reduce the oxygen deficiency. The SEM image of ZnO particles is shown in Figure S1a in the Supporting Information (SI). Optical absorption and emission spectra of as-snthesized ZnO are given in the SI (Figure S1b,c) Device Fabrication. To fabricate the heterojunction, WS2 QDs blended PEDOT-PSS solution was spin-casted on the thin film of ZnO nanoparticles coated ITO substrates at 1000 rpm for 1 min and dried at 90 °C for overnight. Finally, front Al metal electrodes of area 0.25 cm2 were evaporated through a shadow mask by a thermal evaporation technique at a base pressure of 1 × 10−6 Torr. Physical Characterization. Absorption spectra of WS2 dispersion in DMF solution were recorded using a fiber-probe-based UV−vis−NIR charge coupled device (CCD) spectrophotometer. Photoluminescence (PL) spectra of WS2 in DMF solution and heterojunction devices were measured at room temperature using a diode laser (Model EPL 375, Edinburg Instruments, Life Spec II) of wavelength 376.6 nm. The chemical composition of as-exfoliated WS2 was studied using X-ray photoelectron spectroscopy (PHI 5000 Versa Probe II, ULVAC−PHI, INC, Japan) with an incident Al Kα X-ray of energy hν = 1486.6 eV. The surface morphology and thickness of WS2 flakes were investigated using an atomic force microscope (AFM), model No VeecoNanoscope-III. High-resolution transmission electron microscopy (HRTEM) (Model
EXPERIMENTAL SECTION
Materials and Characterization. All the chemicals (WS2) and solvents (hexane, toluene, isopropanol, dimethylformamide (DMF)) were purchased from Sigma-Aldrich and used without further purification. Synthesis of 2D and 0D WS2 Nanostructures. First, 2D WS2 sheets were synthesized from bulk WS2 powder (99%,
