CC BY
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Original article
UDC 544.778.4:546.775
DOI: https://doi.org/10.18721/JPM.16103
NANOSIZED VERTICAL NANOSHEETS MADE OF MOLYBDENUM DISULPHIDE: ELECTRICAL AND OPTOELECTRONIC PROPERTIES
S. E. Alexandrov , Y. Khattab
Peter the Great St. Petersburg Polytechnic University, St. Petersburg, Russia и [email protected]
Abstract. In the paper, the results of the studies in the electrical and optoelectronic properties of vertical sheets made of MoS2 have been presented for the first time. These objects are characterized by a high surface area, exposed edges, reasonable carrier mobility values and high light absorptance. Samples with an average size of about 150 nm were grown by the one-stage metal-organic chemical vapor deposition (MOCVD) technique. Vertically oriented MoS2 sheets were investigated using the scanning electron and X-ray photoelectron microscopy, the X-ray diffraction and Raman spectroscopy. VI characteristics of the samples were obtained as well. Optoelectronic properties of the samples were studied using an argon laser (operates at a wavelength of 513 nm) with a mechanical light modulator. An analysis of the obtained results allows us to state that the studied V-MoS2 sheets should be considered as a very promising material for optoelectronics needs.
Keywords: chemical vapor deposition, molybdenum disulphide, vertical nanosheet, optoelectronic and electrical properties
For citation: Alexandrov S. E., Khattab Y., Nanosized vertical nanosheets made of molybdenum disulphide: Electrical and optoelectronic properties, St. Petersburg State Polytechnical University Journal. Physics and Mathematics. 16 (1) (2023) 24—32. DOI: https://doi. org/10.18721/JPM.l6l03
This is an open access article under the CC BY-NC 4.0 license (https://creativecommons. org/licenses/by-nc/4.0/)
Научная статья
УДК 544.778.4:546.775
DOI: https://doi.org/10.18721/JPM.16103
ЭЛЕКТРИЧЕСКИЕ И ОПТОЭЛЕКТРОННЫЕ СВОЙСТВА НАНОРАЗМЕРНЫХ ВЕРТИКАЛЬНЫХ НАНОЛИСТОВ, ИЗГОТОВЛЕННЫХ ИЗ ДИСУЛЬФИДА МОЛИБДЕНА С. Е. Александров Ю. Хаттаб
Санкт-Петербургский политехнический университет Петра Великого, Санкт-Петербург, Россия и [email protected]
Аннотация. В статье впервые представлены результаты исследования электрических и оптоэлектронных свойств вертикальных листов, изготовленных из дисульфида молибдена МоБ2 (У-МоБ2). Этим объектам свойственны большая удельная поверхность, открытые края, разумные значения подвижности носителей и высокий уровень поглощения света. Образцы со средним размером около 150 нм были выращены методом одностадийного ХОГФ при использовании металлорганических исходных соединений (МОСУО). Листы У-МоБ2 изучены методами сканирующей электронной и рентгеновской фотоэлектронной микроскопии, рентгеновской дифракции и спектроскопии комбинационного светорассеяния. Получены вольтамперные характеристики образцов.
© Alexandrov S. E., Khattab Y., 2023. Published by Peter the Great St. Petersburg Polytechnic University.
Оптоэлектронные свойства У-Мо82 исследованы с помощью аргонового лазера (длина волны — 513 нм) с механическим модулятором света. Анализ полученных результатов позволяет утверждать, что изученные листы У-МоБ2 следует рассматривать как весьма перспективный материал для нужд оптоэлектроники.
Ключевые слова: ХОГФ, дисульфид молибдена, вертикальный нанолист, оптические и электрооптические свойства
Ссылка при цитировании: Александров С. Е., Хаттаб Ю. Электрические и оптоэлектронные свойства наноразмерных вертикальных нанолистов, изготовленных из дисульфида молибдена // Научно-технические ведомости СПбГПУ. Физико-математические науки. 2023. Т. 16. № 1. С. 24-32. БО1: https://doi.org/10.18721/ 1РМ.16103
Статья открытого доступа, распространяемая по лицензии СС БУ-МС 4.0 (ИМ^:// creativecommons.Org/licenses/by-nc/4.0/)
Introduction
Among the family of transition metal dichalcogenides (TMDs), molybdenum disulphide MoS2 is one of the most extensively studied materials due to attractive properties of its thin films, such as possibility to transform band structure from an indirect bandgap to a direct one by decreasing their thickness from bulk to a single layer [1, 2]; high room-temperature carrier mobility in MoS2 (was measured to be about 200 cm2 -V"us-1) with a large switching on/off ratio exceeding 108 value, strong interaction with light and low energy consumption [1 — 5]. Consequently, MoS2 has attracted considerable interest as a promising candidate for manufacturing enhanced transistors, sensors, photodetection and electronic displays [5 — 8]. In addition, MoS2 has a promising outlook in the fields of solar cells, energy storage, energy conversions and catalytic applications. For example, MoS2 can be used as a highly efficient electrocatalyst for hydrogen evolution reaction [9 — 14].
Most of the papers published during the last decade have been devoted to formation of mono-or few- layers planar MoS2 structures deposited on the surface of the substrate (mainly sapphire or silicon oxide). However, recently the deposition of vertically aligned sheets of MoS2 has been achieved and remarkable interest appeared in them due to their specific features including maximum surface area and extensively exposed edges [15 — 17].
The vertical MoS2 sheets have a complicated structure that has many dongle bonds, in comparison to the layers grown horizontally on the substrate surface. Although many research groups have reported an formation of the MoS2 vertical nanosheets, their electrical and optoelectronic properties have not been fully studied yet.
The purpose of this work was to make nanosized vertical sheets of MoS2 (V-MoS2) and to study their electrical and optoelectronic properties.
Materials and methods
The deposition process was carried out at a low pressure in a hot-wall horizontal tube reactor with a diameter of 56 mm and a length of about 300 mm made of quartz. Mo(CO)6 powder and H2S gas were used as precursors for metal-organic chemical vapor deposition (MOCVD) to grow MoS2 films. The molybdenum containing the precursor Mo(CO)6 was introduced into the deposition chamber from the evaporator maintained at the temperature of 30 °C by using argon as a carrier gas. To ensure the complete transfer of the precursor into the reactor, the vapor transport lines were maintained at about 120 °C. The total pressure in the reaction chamber was set to approximately 70 Pa, the substrate temperature was approximately 550 °C and the deposition time was 30 min. The substrates (silicon wafer, silicon wafer with deposited 100 nm SiO2 film, fused quartz) were cleaned in acetone, alcohol and deionised water for 10 min.
The morphology and composition of the deposited films were studied with the use of scanning electron microscopy (Supra 55 VP with WDX and EDX spectrometers). X-ray photoelectron spectroscopy (SPECS HAS 3500) was used for chemical analysis. The presence of crystalline phases was investigated using Х-ray diffraction (Super Nova Dual Wavelength
© Александров С. Е., Хаттаб Ю., 2023. Издатель: Санкт-Петербургский политехнический университет Петра Великого.
(Agilent Technology), Cu K (k = 1.5405 A)) and Raman spectrum was measured by Raman spectrometer (Horiba 800).
Silver electrodes were deposited on the samples by electron beam evaporation using high vacuum system (10-6 Torr) and electrical properties were measured by Keithley 237 under pressure of about 10-4 Torr in Janis cryostat. Voltage-current (VI) measurements were carried out on three samples (repeated 5 times for each sample), and then the measurement results were averaged. Optoelectronic properties were measured using an argon laser (green light) with mechanical chopper.
Results and discussion
Structure and composition. The scanning electron microscopy (SEM) images of the deposited film made on the surface of SiO /Si substrate are presented in Fig. 1. As can be seen, the film consists of sheets grown perpendicular to the surface of the substrate. The sheet sizes vary and their average value is about 150 nm. The cross-section of the layer is shown in the inset of the same figure and it clearly shows the vertical growth of the sheets with a height of about 250 nm. Films deposited on the silicon and quartz substrates were characterized by similar morphology without any noticeable differences.
Crystal structure of the films was studied using powder X-ray diffraction (PXRD). As Fig. 2 suggests, all reflections can be attributed to the pure hexagonal phase of MoS2 with the following
lattice parameters:
a = 3.161 A and c = 12.299 A
(standard file JCPDS No. 37-1492) and diffraction peaks from crystalline impurities were not observed. Since the X-ray spectrometer had "powder geometry", it was possible to observe reflections only from planes parallel to the substrate. In this connection, it seems highly probable that the strong reflection (00L) comes from the horizontal layer, while reflections (100) and (101) can refer to vertical sheets. Using the Scherer formula, the thickness of the horizontal layer was calculated from the Fig. 1. SEM image (top-view) of vertical MoS2 FWHM value of the diffraction line (002). The nanosheets: their sizes vary and the most are around minimum thickness of the horizontal layer was
150 nm. Inset: the cross section of the objects found to be about 37 nm.
Fig. 2. PXRD pattern of the MoS2 film with vertical nanosheets on the silicon oxide substrate with peaks indexed
The Raman spectroscopy is widely used to study crystal structure, the quality of MoS2 substance, the number of monolayers and texture of MoS2 films. Two strong characteristic Raman modes EUg and A of MoS2 were observed in the Raman spectra of the deposited films with vertical nanosheets at 381 cm-1 and 407 cm-1, corresponding to in-plane vibration of molybdenum and sulfur atoms, and out-of-plane vibration of sulfur atoms, respectively (Fig. 3). Both modes show a red shift of about 1 cm-1 comparing to the values typical for the bulk MoS2 probably due to strain in the films. The frequency difference between the E12 and A Raman modes is about 26 cm-1, which indicates the presence of around seven or more layers in the MoS2 nanosheets.
The energy dispersive X-ray (EDX) and X-ray photoemission (XP) spectra of the film consisting of vertical sheets have signals from Mo, S, C and O atoms only. It seems very likely that the presence of oxygen and carbon is caused by their adsorption from the surrounding
atmosphere. The composition of the MoS, film is close to stoichiometric and it has been separately confirmed by the results of XP and EDX spectroscopies (the S/Mo ratio is about 2.01). High resolution XP spectroscopy analysis was carried out to investigate the chemical states of Mo atoms. The deconvolution of the Mo 3d core spectrum by peak fitting reveals two Mo 3d doublets (see Fig. 4). The Mo-atom signal mainly arises from Mo 3d5/2 (228.9 eV) and Mo 3d3/2 (232.1 eV) characterized the molybdenum Mo4+ sulphide components. The small doublet at lower binding energy (233.0 and 230.2 eV) can be related to defects or 1T phase of MoS2, while the doublet at higher energy (233.4 and 235.7 eV) corresponds to higher oxidation states (Mo5+) and (Mo6+) due Fig. 3. Raman spectrum pattern of MoS2 film to the presence of molybdenum oxide and with vertical nanosheets on silicon oxide substrate defects.
Fig. 4. XP spectrum of MoS2 layer, high resolution spectrum of Mo 3d components with fitting (the red line is an experimental datum and the black one is a fitting result)
Thus, the results of the XRD analysis suggest that the vertical nanosheets are formed not directly on the substrate surface, but on the surface of the horizontally grown MoS2 layer with a certain thickness. That issue has already been raised in our Ref. [18].
Electrical and optoelectronic properties. Silver films were used as the contact layer to the MoS2 ones (the films were deposited on the SiO2/Si substrates) since the ohmic nature of the contact can theoretically be expected considering the fact that Ag and MoS2 have work function values of 4.5 eV and 5.2 eV, respectively. Voltage-current (VI) relationships measured for vertical sheets under vacuum are shown in Fig. 5. From this figure we notice that they are not linear.
Several models including the Schottky emission, direct tunneling, the Poole — Frenkel emission, and space-charge-limited-current (SCLC) state [19] were used for fitting them with experimental curves, and it was found that the Lampert theory of SCLC could be successfully used to explain the current behavior. As can be seen from Fig. 5, the VI curve plotted in the (lgV — lgl) coordinates has four distinct regions: ohmic where I ~ V (region I), the Mott — Gurney's law I ~ V3 2 (IV), and the trap-filled limit voltage (VTFL) (II). The III region corresponds to the transition from the trap-limited conduction to the trap-free one. VON and V are the voltage values for the transition from ohmic conducting to the Mott — Gurney's law and from trap-filled conducting to the Mott — Gurney's law, respectively.
100
10
o 0.1
0.01
/A 1
■33 20
1 f
? A
c D QOO_________r r u
! I
3 / 4
\ "-o.cra / J
6C04. / J
V™ jff
^QN
a
f
1 II til iv ;
* "* if r»" i
0.1 1 10
Voftage^V
Fig. 5. Current density plotted as a function of the applied voltage on the logarithmic scale, insert: the IV region for vertical sheets on linear scale.
All measurements were made under vacuum
Using the SCLC method made it possible to calculate a lot of important transport parameters including the carrier mobility, the concentration of free charge carrier and the trap density. Based on the SCLC model for thin films and using the procedure described in Ref. [19], the electrical properties of vertical sheets of MoS2 were calculated and tabulated.
Molybdenum disulphide MoS2 is a material with moderated carrier mobility and the highest values usually correspond to the monolayers (the maximum value reported is about 200 cm2/(V-s) [1]). It should be noted that the mobility value of about 45 cm2/(V-s) measured in this work for vertical sheets is within the typical range. However, after filling the traps, a noticeable increase in the mobility is detected by more than an order of magnitude (region IV), demonstrating a strong effect of defects on mobility in MoS2. Such value has to be considered as rather high taking into account the low deposition temperature (550 °C) of MoS2 nanosheets and the complicated structure of the sheets. To confirm this high value of the carrier mobility time-dependent photoresponses were measured by exposing vertical sheets to the argon laser light (513 nm) with 10 V bias. Time responses were measured by real-time CW laser on/off using a variable speed-controlled mechanical chopper. The fulling time t was estimated by fitting with exponential function
y - y0=^exP [(x0 - x)/tl (1)
where x0 and y0 are the initial values, i. e. the time and current values for the moment when the light was turned off.
As can be seen from Fig. 6, the measured values of the rise time (0.65 ms) and the fall time (0.69 ms) are quite small and agree well with the relaxation time calculated using the SCLC method (see the Table).
Table
Electrical properties of MoS2 vertical sheets calculated from SCLC curves (see Fig. 5)
Parameter Unit Curve region used Parameter value
Carrier mobility cm2/(V^s) II 31.4
IV 475.6
Carrier concentration (in thermal equilibrium) cm-3 Border I - II 1.0759e+16
Density of traps Border II - III 6.673e+16
Dielectric relaxation s II 1.41e-2
time IV 7.30e-4
Footnotes. 1. The calculations were performed using the procedure given in Ref. [19]. 2. The dielectric relaxation time was obtained using the calculated carrier mobility as the base.
Fig. 6. Photoresponse curve of vertical nanosheets measured using a CW laser with mechanical chopper.
The fulling time was estimated by fitting with exponential function (1)
Summary
The results obtained allow us to conclude that V-MoS2 nanoscale vertical sheets should be considered as a promising material for the needs of optoelectronics. The study carried out has been demonstrated that MoS2 nanosheets are able to provide high mobility and fast optoelectronic response.
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THE AUTHORS
ALEXANDROV Sergey E.
Peter the Great St. Petersburg Polytechnic University 29 Politechnicheskaya St., St. Petersburg, 195251, Russia [email protected] ORCID: 0000-0003-0129-0479
KHATTAB Yossef
Peter the Great St. Petersburg Polytechnic University 29 Politechnicheskaya St., St. Petersburg, 195251, Russia [email protected] ORCID: 0000-0001-6116-7830
СВЕДЕНИЯ ОБ АВТОРАХ
АЛЕКСАНДРОВ Сергей Евгеньевич — доктор химических наук, профессор Высшей школы физики и технологий материалов Санкт-Петербургского политехнического университета Петра Великого, Санкт-Петербург, Россия.
195251, Россия, г. Санкт-Петербург, Политехническая ул., 29
ORCID: 0000-0003-0129-0479
ХАТТАБ Юссеф — аспирант Высшей школы физики и технологий материалов Санкт-Петербургского политехнического университета Петра Великого, Санкт-Петербург, Рос-
195251, Россия, г. Санкт-Петербург, Политехническая ул., 29
ОКСГО: 0000-0001-6116-7830
Received 20.12.2022. Approved after reviewing 22.12.2022. Accepted 22.12.2022. Статья поступила в редакцию 20.12.2022. Одобрена после рецензирования 22.12.2022. Принята 22.12.2022.
© Санкт-Петербургский политехнический университет Петра Великого, 2023
