Subscriber access provided by UNIV TEXAS SW MEDICAL CENTER
Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
Synthesis and Property of Copper Impregnated #-MnO2 Semiconductor Quantum Dots Dheeraj Mondal, Biplab Kumar Paul, Santanu Das, Debopriya Bhattacharya, Debopriyo Ghoshal, Papiya Nandy, Kaustuv Das, and Sukhen Das Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01745 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
Synthesis and Property of Copper Impregnated α-MnO2 Semiconductor Quantum Dots
2
Dheeraj Mondal a, Biplab Kumar Paul b, Santanu Das a, Debopriya Bhattacharyac, Debopriyo Ghoshalc,
3
Papiya Nandy d, Kaustuv Das a, Sukhen Das *a
4
a Department
5
b CSIR-Central
6
c Indian
7
d
8
Corresponding Author
9
*Email:
[email protected] 10
of Physics, Jadavpur University, Kolkata-700 032, India. Glass and Ceramic Research Institute, Kolkata-700 032, India.
Institute of Engineering Science and Technology, Shibpur, Howrah-711 103, India
Centre For Interdisciplinary Research and Education, Kolkata- 700068, India.
ORCID: Sukhen Das: 0000-0001-8372-3076
11 12
ABSTRACT
13
Because of the superior optical and electrical properties, copper impregnated size tuneable
14
high-temperature stable manganese dioxide semiconductor quantum dots (SQDs) have been
15
successfully synthesized by a modified chemical synthesis technique. Their size-dependent dielectric
16
properties, semiconducting properties, and current-voltage (I-V) characteristics have been investigated.
17
X-ray diffraction pattern and Raman spectra (RS) confirmed that the required phase is present. Due to
18
the different sintering temperature tuneable size of SQDs has been found and confirmed by high
19
resolution transmission electron microscopy (HRTEM). The band gap energy of the material is found
20
to be 1.25-1.67 eV, measured from Tauc plot using UV-vis absorbance spectrum and their
21
semiconducting properties have been confirmed by the non linear current-voltage (I-V) behaviour.
22
Most intense green emission peak of photoluminescence (PL) spectroscopy confirms the oxygen
23
vacancy defect state. The stoke shifting of RS, UV absorption and PL emission are the footprint of
24
quantum confinement effect. Incorporation of a little amount of Cu in tetragonal hollandite structure of 1 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 36
1
α-MnO2 generates strain within that structure. This leads to create sufficient crystal defect state as well
2
as rise in dielectric constant accompanied with low dielectric loss and higher ac conductivity. All these
3
highly desirable properties make the SQDs a potential candidate for developing multifunctional photo-
4
electronic devices. Owing to the tuneable bandgap and electronic transport of the SQDs, we realized
5
that the controllable size paves the way for designing SQDs possessing unique properties for optical
6
and electronic device applications. Using this material as a high dielectric separator, a high
7
performance supercapacitor has been successfully fabricated which can able to light up 15 number of
8
LEDs for 47 min 23 sec after charging it only for 30 seconds.
9 10
KEYWORDS: α-MnO2, Semiconducting Quantum Dot, Size tuneable band-gap, dielectric properties,
11
Opto-Electronic device
12 13
INTRODUCTION
14
Due to the achievement of ever smaller dimensions and dramatical changes in structural and
15
optical properties, semiconductor quantum dot1,2,3 particles (SQDs) make itself as most exciting and
16
leading topics in the field of research. All of this excitement about nanoscience and nanotechnology
17
originates due to the multitude of applications in advanced optoelectronic devices like lasers4, optical
18
amplifiers2,4, signal detector4,5,6, quantum information processing7, light emitting devices8, photovoltaic
19
sources9, photocatalysis10, solar energy conversion9, nanophotonics11, smart catalysts12,13, biomedical
20
applications14 and so on. The essential physical and chemical properties of SQDs which depend on
21
their ever smaller size, make themselves a possible candidate to engineer the material properties as per
22
our demand not only by tailoring the size and shape of the SQDs but also by defining its chemical
23
composition and the way in which individual building blocks are assembled.
2 ACS Paragon Plus Environment
Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
Tremendous efforts are being given by the worldwide researchers to tailor the size of SQDs to
2
very few nanometres with the dramatic impact of quantum confinement effect and their high-
3
temperature stability15. Because the synthesis process is highly susceptible towards degradation by
4
temperature, light, oxygen, and moisture due to the surface oxidation tendency, SQDs are lowering the
5
yield of desired QDs with concomitant formation of metal oxides and thereby deteriorate the quality
6
which results in the gradual decay of their interesting properties to a great extent. On the other hand, at
7
high-temperature SQDs3 stability is a worldwide requirement for various applications but still has been
8
universally a great challenge among the scientific research community. Several strategies have been
9
developed for synthesizing QDs at high temperature with long-term air, chemical and mechanical
10
stability. But they are not sufficient to meet the global requirements. So, till now it is a great challenge
11
to synthesize and thereby use them in electrical4,5,6,9 and optoelectronic4,8 industrial application.
12
Due to the wide variety of crystallographic polymorphs and different shapes with different
13
microstructure, MnO2 SQDs have gained tremendous attention for some decades by the researchers to
14
reduce the size of the SQDs but still has been a great challenge to make the tuneable optical and high-
15
temperature electrical applications. Most of the MnO2 SQDs networks showed a high specific surface
16
area, high surface to volume ratio and typical size ~7 nm, which are the main building blocks of high
17
charge storage supercapacitor, high-capacity lithium-ion batteries, lithium-air batteries, redox catalysts,
18
and so forth compared to other transition metal oxide SQDs. Previous experimental
19
studies16,17,18,19,20,21,22 imply that charge storage performance of MnO2, significantly depend on its
20
morphology and size13,23,24, i.e. on specific surface area. More preciously to say, if particles are
21
synthesized in QDs form, its specific surface area is significantly enhanced.
22
On the other hand, having large quantum yield, a very sharp density of states, excellent
23
transport phenomenon, higher emitted photons, SQDs are the promising candidate for more efficient
24
photovoltaic solar cell application9. The variation of the band gap of the SQDs depends on the size1,2,3, 3 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 36
1
makes it easy to tune the absorption and emission spectra1,3 which is highly desirable for the engineered
2
optical application.
3
In the preceding work, we have adopted a novel approach to synthesis the high temperature
4
sustainable α-MnO2 SQDs which can be the most promising next-generation advanced material for
5
optoelectronic industrial applications. We have investigated the modification of its electrical and
6
optical behaviour with the incorporation of foreign element like copper.
7 8
EXPERIMENTAL SECTION
9
The X-ray diffraction pattern for observation of crystalline phase of all samples were recorded
10
using X-ray diffractometer (model-D8, Bruker AXS, Wisconsin, USA) with Cu Kα radiation-1.5418 Å
11
and operating voltage 40 KV with a scan speed of 0.3 s/step in the range of 2θ from 20° to 80°. The
12
characteristic vibrational stretching and bending modes of Mn-O-Mn chain of the prepared samples
13
were studied by RT Raman spectroscopy (model-Newport RS 2000TM). Field emission scanning
14
electron microscope (FE-SEM) (ZEISS Sigma 300) was employed for the morphological study.
15
Particle size, fringe pattern and EDX were investigated using High-Resolution Transmission Electron
16
Microscope (HR-TEM) (JEOL, model no. JEM-210, USA). The optical absorption (UV-Visible)
17
spectrum of the QD particles was taken in the range of 200-1000 nm using Lamda 365 UV-Vis
18
spectrophotometer, Perkin Elmer, Germany. Current-voltage characteristics were analyzed using
19
KEITHLEY 2430-Source Meter. Dielectric measurement and electrical conduction mechanism were
20
carried out using Agilent 4294A Precision Impedance Analyzer in the frequency range 40 Hz to 10
21
MHz. Before doing the dielectric measurements, all the samples were pelletized using a hydraulic
22
pelletiser with the pressure of 75 kg/cm2 and then coated by silver paste. The photoluminescence
23
emission spectrum was recorded by using Cary Eclipse Fluorescence Spectrophotometer, Agilent
24
Technology with an excitation wavelength (λex) of 276 nm. 4 ACS Paragon Plus Environment
Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
Synthesis of Cu-doped α-MnO2 SQDs: Cu impregnated α-MnO2 SQDs were successfully synthesized
2
by modified chemical synthesis technique. In a typical synthesis procedure, 0.25 vol% of Polyvinyl
3
alcohol (PVA) dissolved in 200ml double de-ionized water. Then separately prepared 0.005 (M)
4
Mn(CH3COO)2, 0.01 (M) KMnO4 and 0.0001 (M) Cu(CH3COO)2 of 100 ml each were mixed together
5
under continuous stirring for 3 hours at 450 rpm. Then previously prepared PVA solution was added
6
with the mixture under vigorous stirring at 950 rpm at room temperature for 36 hours. The pH of the
7
solution was maintained at 10 by addition of KOH. Then the resulting solution was kept at undisturbed
8
condition at room temperature for complete reaction for another 24 hours. The precipitation was
9
separated and collected by centrifugation at 20000 rpm for 20 minutes followed by alternately
10
centrifuging several times with double de-ionized water and ethanol. Collected brown precipitates were
11
first dried in vacuum oven at 80°C for overnight. Finally the sample was annealed at 300 °C, 450 °C
12
and 600°C in a muffle furnace at 1°C/minute increasing temperature with 2 hrs of holding time. The
13
cooling rate was also 1°C/minute. Sintered samples are labelled as CMO 300, CMO 450 and CMO 600
14
respectively.
15 16
RESULTS AND DISCUSSION
17
X-ray diffraction pattern of Cu-doped α-MnO2 sintered at three different temperatures 300 °C,
18
450 °C and 600 °C have been shown in Fig. 1(a). All diffraction peaks have been successfully assigned
19
with JCPDS card no. #44-0141 which implies the tetragonal structure23,25,26 of MnO2 with the space
20
group I4/m23,27 and match well with that of α-MnO2 indicating that the nanoparticles are crystallized in
21
the (2×2) tunneled hollandite-type network23,28,29. The large tunnel cavity and octahedral space of
22
MnO2 facilitate effective doping20,30 in MnO2. No additional peak related to any other phase of
23
manganese or copper was noticed which implies the formation of pure α-MnO2 in modified chemical
24
synthesis route. The average nanocrystalline diameter of all the samples was calculated using Debye5 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 36
1
Scherrer31,32 equation from the broadening of the most intense peak (211) present in the corresponding
2
diffraction pattern, shown in table (1).
3
Kλ
< 𝐷 > (211) = β1/2 cos θ
(1)
4
Here, D is the average nanocrystalline size, λ is the wavelength of the incident Cu-K beam (λ
5
=1.54Å), θ is the corresponding Bragg’s angle, β1/2 is the full width at half maximum (FWHM) of the
6
(211) peak and K is the shape factor having the constant value of 0.9.
7 8
Figure 1. Phase and Structural Model: (a) XRD pattern of CMO 300, CMO 450 and CMO 600 (b)
9
rescale portion of (211) peak of the entire sample (c) Characteristics Raman vibration mode of the said
10
samples (d) Proposed model of Cu-doped α-MnO2.
11
The average nanocrystalline diameter (D) of the samples sintered at different temperatures were
12
estimated after broadening the (211) peak ( shown in Fig. 1(b) ) and it has been found that the size
13
increases from 6.94 nm (for CMO 300) to 19.24 nm
(for CMO 600) with increasing sintering
6 ACS Paragon Plus Environment
Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
temperature. Actually, the copper incorporation procedure reduces the MnO2 nuclear formation and
2
delay in further growth rate which results in a very lower particle dimension33. The lattice parameters
3
of the tetragonal lattice (a=b≠c) are calculated from the relation, 1
4
2
𝑑
=
ℎ2 + 𝑘2 𝑎
2
𝑙2
+ 𝑐2
(2)
5
Here, h, k, and l are the Miller indices, respectively and d is the interplanar spacing of the plane (hkl)
6
calculated using the Bragg's equation34.
7
2𝑑 sin 𝜃 = 𝑛𝜆
(3)
8
Where ‘n’ is the order of diffraction, ‘λ’ is the wavelength of the X-ray and ‘θ’ is the angle of
9
diffraction. The refined lattice constants were calculated from the above equation and have been shown
10
in the Table (1). All the values are in a good agreement with those reported in the earlier experimental
11
studies23,30. It is quite clear that the crystal of α-MnO2 prefers the growth in the [00l] direction, which is
12
due to loosened lattice constraints in the nanostructure only in that direction. The volume of the unit
13
cell of the tetragonal structure of Cu-doped α-MnO2 nanoparticles have been calculated using the
14
equation
15
𝑉 = 𝑎2 × 𝑐
(4)
16
and are given in the Table (1).The corresponding value of microstrain (ε)35 of the sintered Cu-doped
17
MnO2 nanoparticles are calculated using the formula
18 19
𝛽ℎ𝑘𝑙
𝜀 = 4 tan 𝜃
(5)
20
Here microstrain of all samples is calculated using the most intense peak (211) present in the XRD-
21
pattern. Due to the higher annealing temperature, the volume of nanocrystal gets increased which
22
induces sufficient lattice distortion in the crystal structure. The ionic radius of Cu (0.73Å) being quite
23
high compared to that of manganese (0.46Å), when it is implanted within MnO2 lattice structure, some
24
strain along with vacancy defects are generated. Dislocation density (δ) represents the amount of defect 7 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 36
1
present in the samples, which is defined as the length of dislocation lines per unit volume of the crystal
2
and has been calculated using the equation 1
3
(6)
𝛿 = 𝐷2
4
Table 1. Sample Specifications: Structural parameters i.e., Lattice parameters (a, b, c), Crystallite size
5
(D), Unit cell volume (V), Microstrain (ε) and Dislocation density (δ) Sample ID
Lattice Parameter (Å) a=b
c
Crystallite
Unit Cell
Microstrain
Dislocation
Size (D)
Volume (V)
ε (~10-3)
Density
(nm)
(Å3)
δ (~1015)
CMO 300
9.8465
2.8419
6.94
275.53
8.213
20.8
CMO 450
9.8465
2.8551
13.89
276.81
4.103
5.18
CMO 600
9.8465
2.8582
19.24
277.11
2.961
2.70
6 7
This analysis indicates that all the prepared samples contain Cu- inside the (2×2) tunnel of α-MnO2
8
hollandite-type network. All the calculated parameters have been mentioned in Table (1).
9
Raman scattering (RS) spectroscopy provides useful alternatives and/or supplements to X-ray
10
diffraction (XRD) for structural evaluation of materials. The lattice vibrational features of RS-
11
spectroscopy yield a more complete, reliable description and different local structural properties of the
12
material as well as crystalline disorders or defects of the MnO2 related compound. Structurally, α-
13
MnO2 type materials crystallize into a body-centered tetragonal structure, space group I4/m23,27. The
14
factor group analysis indicates that (6Ag +6Bg +3Eg) spectroscopic species are Raman active36,27 due to
15
the polycrystalline nature of the analyzed sample. The RS spectrum of Cu-doped α-MnO2 NP displays
16
three main bands located at 178, 566 and 631cm-1 along with four broaden peaks at 270, 353, 380 and
17
471 Cm-1 respectively (for CMO 300) as shown in Fig. 1(c). All bands can be assigned to the vibrations 8 ACS Paragon Plus Environment
Page 9 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
of Mn-O bands within MnO6 octahedra of the α-MnO2 lattice27. The low-frequency Raman band at 176
2
cm-1 is assigned to an external vibration that derives from the translational motion27,37 of MnO6
3
octahedra, while the other RS bands are due to the internal modes, i.e, bending and stretching species27.
4
The two high frequency of RS bands at 566 cm-1 and 631 cm-1 belongs to Ag spectroscopic species that
5
originate from orthogonal vibrations of the Mn-O bond along the direction of MnO6 octahedra within a
6
tetragonal hollandite-type framework36,38 ; the relative intensity of the two Ag modes are correlated to
7
the nature of the tunnel species. The variation of less intense peaks i.e. 270, 353, 380 and 471 cm-1 (for
8
CMO 300) confirms the phonon density of state rather than Raman-allowed zone center phonons,
9
which is due to the confinement of phonons by crystal defects and local lattice distortions in the as-
10
prepared Cu doped α-MnO2 QDs39. The observed red shift of intense Raman peak position is a
11
consequence of quantum confinement phenomenon40.
12
The field emission scanning electron microscopy (FESEM) technique was used to investigate the
13
morphology of the synthesized powders namely CMO 300, CMO 450 and CMO 600, shown in Fig.2
14
(a), (b) and (c) respectively. Fig. 2(d) reveals the desire elemental composition of as synthesized
15
sample.
9 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 32 33 34 2 35 36 3 37 38 39 4 40 41 5 42 43 44 6 45 46 7 47 48 8 49 50 9 51 52 53 10 54 55 11 56 57 58 59 60
Page 10 of 36
Figure 2. Microstructure and Quantitative Elemental Analysis: FESEM microstructure image of (a) CMO 300, (b) CMO 450 and (c) CMO 600 and (d) EDX spectra of the as-prepared sample. All the representative microstructures illustrate that the samples consist of nanosized CMO QD crystals with a little crystal size variation followed by sintering temperature. The top surface view of the microstructure of the CMO 300 QDs demonstrates that at 300OC the particle morphology is irregular and spherical shape whereas morphological behaviour has been changed when sample calcined at higher temperatures. With the increase of sintering temperature, the C-axis length has been found to elongate, which results in an increase in unit cell volume as well as the particle size. If we observe FESEM micrograph more carefully, higher calcination temperature affects sufficient grain growth as well as higher crystallinity which reduce the particle agglomeration. 10 ACS Paragon Plus Environment
Page 11 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
The projected atomic potential view of the CMO 450 SQDs was obtained by high-resolution
2
transmission electron microscopy (HRTEM) measurement using carbon coated copper grid, exhibited
3
in Fig. 3. The energy dispersive X-ray spectroscopy (EDX) has been analyzed during the HR-TEM
4
experiment which provides the information about the elemental composition. The EDX was performed
5
at 200KV accelerating voltage and spectrum is collected from the top view of the particles of dipping
6
cycle 6 by using fast-moving electrons X-ray Signals and presented in Fig. 2(d). The EDX clearly
7
confirms the existence of manganese (Mn), oxygen (O2) and copper (Cu), which are the main elements
8
of CMO QD particles. The ratio of the peaks is in the good agreement with our expected elemental
9
composition. The representative image demonstrates that the nearly monodisperse, nanosized CMO
10
SQD crystals with the diameter of ~7 nm (estimated through Gaussian distribution) are well crystalline
11
with very low agglomeration. The particles are mostly spherical in shape and size histogram obtained
12
from each of the samples was found to confirm the size of our particle in QDs range and which is more
13
crucial for the occurrence of quantum confinement phenomenon. Images of isolated nanocrystals at
14
higher magnification (HRTEM, Figure inset) further confirm the crystallinity and phase purity of the
15
synthesized CMO SQD. The well-defined two-dimensional lattice fringes of QD nanocrystal indicate
16
good crystallinity41 of our material. The measured interplanar distance is 0.14 Å (shown in Fig (3(b),
17
Inset)), is in good agreement with the calculated, interplanar spacing (d200) obtained from XRD pattern
18
of CMO 450 sample (using equation 2).
11 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 12 of 36
1 2
Figure 3. QD overview: (a) HR-TEM image of CMO 450 QD and corresponding size histogram (inset).
3
The Gaussian distribution shows the range of particle diameter around 7 nm (b) Fringe pattern at 2 nm 12 ACS Paragon Plus Environment
Page 13 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
scale range shows well crystalline nature and measured interplanar separation at (200) plane is 0.14 nm
2
(Inset). HR-TEM image with size histogram of (c) CMO 300 and (d) CMO 600. (e) EDX spectra of as
3
synthesized sample.
4
In the present experiment, the formation of α-MnO2 SQD is also attributed to PVA as surface coating
5
agent after the nucleation. At higher sintering temperature, decomposition of PVA plays the main role
6
of structure-directing and facilitates such low dimensional growth of the CMO SQD crystals.
7
To investigate the optical properties and band-gap of α-MnO2 SQDs of different temperatures,
8
ultraviolet-visible (UV-vis) absorption spectroscopy has been carried out and demonstrated in Fig. 4(a).
9 10
Figure 4. Light Absorbance And Band Gap Properties: (a) UV-Vis absorption spectra of α-MnO2 QDs
11
(CMO 300, CMO 450 and CMO 600). Indirect band gap energies are plotted using Tauc plot (Inset
12
graph) and (b) Energy band structure with discrete electronic energy levels due to the variation of QD
13
particle size.
14
Broad absorption band ranging between 250 nm and up to 600 nm with the peak position of 288.95 nm,
15
318.45 nm and 324.6 nm for CMO 300, CMO 450 and CMO 600 respectively have been discovered.
16
The intensity of absorbance and position of the peak may vary depending upon the various factors like 13 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 36
1
particle size3,42,43, band-gap3,44, oxygen deficiency, surface roughness, defects31 within grain structure
2
of the nanocrystalline material. The band-gap energies Eg for the Cu-doped α-MnO2 nanoparticles are
3
estimated using the Tauc plot23 relation given below
4 5
𝛼𝐸 = 𝐴(𝐸 ― 𝐸𝑔)2
(7)
6
where, E and Eg are the photon energy and the optical band gap energy in eV, respectively. Further α
7
and A are the absorption coefficient and a constant term respectively. In Tauc plot, (αhυ)1/2 is plotted
8
against photon energy hυ and the indirect optical band gap energy23 is calculated by extrapolating the
9
linear region and intersecting the linear portion of the curve to the energy axis (shown in Fig. 4(a)
10
Inset). Estimated band gap for sample CMO 300 was found to be 1.67 eV and it gradually decreased to
11
1.38 eV (for CMO 450) and 1.25 eV (for CMO 600). Therefore sintering temperature and Cu-doping
12
were found to have a significant role to modulate the band gap energy in our synthesized sample.
13
Observed red shifting45 of band gap energy value corresponds to the occurrence of quantum
14
confinement phenomenon. Our XRD and FESEM analysis reveal that particle size of entire samples is
15
within the range of QD. The observed red shifting of UV and PL peak can be interpreted due to the
16
confinement of exciton in a zero-dimension spherical potential well in the spherical quantum dot. The
17
relationship between band-gap (Eg) and size of QD can be approximately written from Brus equation2
18
ħ2𝜋2
𝐸𝑔∗ ≅𝐸𝑛𝑐 𝑔 + 2𝑒𝑟2
(
1 𝑚𝑒∗
1
)
1.8𝑒2
+ 𝑚 ∗ ― 4𝜋𝜀𝜀0𝑟 ℎ
(8)
19
Where Egnc is the energy gap of the nanocrystal, r is the particle radius; me and mh are the effective
20
mass of the electrons and holes respectively. Further, εr is the relative permittivity, ε0 is the permittivity
21
in free space; ħ is the Planck constant, respectively. Following the above equation, the band-gap
22
decreases with the expanding the particle diameter and also discrete energy levels (with different
23
quantum no) arise at the band edges of both the conduction band and valence band (shown in Fig.
24
4(b)). The UV-absorption peaks of Cu-doped α-MnO2 nanoparticles appeared around 300 nm is 14 ACS Paragon Plus Environment
Page 15 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
attributed to п-п* transition46 between antibonding and bonding MO (molecular orbital). As the
2
molecules become larger with the temperature, the number of AOs (atomic orbitals) that are combined
3
to form MOs (bonding and antibonding) increases, leading to an increasingly larger number of energy
4
levels and decreasing the HOMO-LUMO energy gap47, shown in schematic diagram (Fig 4(b)).
5
Quantum confinement effect leads the semiconducting properties1,2,23 in the nanocrystal which is
6
verified from non-linearity of I-V characteristics curve, shown in Fig. 5.
7
Current-voltage measurement has been performed to study the semiconducting properties of Cu
8
implanted MnO2 SQD nanoparticles. I-V characteristics of sample CMO 300, CMO 450 and CMO 600
9
are recorded at room temperature within the range of (±) 6 Volt, shown in Fig. 5 (a), (b) and (c)
10
respectively.
11 12
Figure 5. Semiconducting property analysis: Non-ohmic behavior of current (I)-voltage (V)
13
characteristics of the samples (a) CMO 300, (b) CMO 450 and (c) CMO 600.
14
Entire sample shows non-ohmic behavior48 and that is more prominent in sample CMO 300 and CMO
15
450. Interestingly our 600°C annihilated sample i.e. CMO 600 exhibits slightly linear behavior. This
16
happened due to enhancing electrical conductivity in cu doped MnO2 when annihilated at 600°C.
17
Similar behavior has been observed and cause behind it is explained in dielectric section. So non-ohmic
18
type of I-V characteristic curve implies our cu doped MnO2 nanoparticles are semiconductor48 in
19
nature. 15 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 16 of 36
1
Dielectric properties of nanomaterials depend on the frequency of the external applied electric
2
field, temperature, chemical composition, grain structure etc. Relative dielectric constants are
3
expressed in terms of complex quantity i.e. ɛ = ε′ + j ε′′, where, ε′ and ε′′ are the real and imaginary part
4
of the relative dielectric constant respectively. The real part contributes to the amount of energy stored
5
in the dielectric material due to polarisation whereas the imaginary part is associated with the energy
6
dissipation within the dielectric materials. Frequency-dependent real part of the dielectric constant can
7
be calculated using the relation
8 9
𝐶𝑑
𝜀′ = 𝜀0𝐴
(9)
10
C is the capacitance of the sample, d and A are thickness and area, respectively of the pellet and ε0 is
11
the free space permittivity (8.85 × 10-12 F/m). Fig. 6(a) shows variations of the real part of dielectric
12
constant (ε′) for entire samples with applied electric field frequency (ranging from 40 Hz to 10 MHz) at
13
room temperature. It showed rapid dispersion behaviour near low-frequency region followed by
14
frequency independent nature towards higher frequency region. The real part of the complex dielectric
15
constant was found to be maximum in sample CMO 450 (2.39×105 at 40 Hz) compared to CMO 300
16
(2.27×104 at 40 Hz) and thereafter decreased again in sample CMO 600 (1.54×104 at 40Hz). Previous
17
experimental studies with different QDs performed by Guan et al.49, Kumar et al.50 and Chen et al.51
18
have reported dielectric values ~60, ~190 and ~105 respectively. Our experimental data was found
19
more superior with comparison of those previous records. So this type of high dielectric material can be
20
used as a high-performance separator in super capacitor. Room temperature AC conductivity of the
21
samples was determined and has been depicted in Fig. 6(b). It has been found that up to 104 Hz
22
frequency of applied alternating electric field the ac conductivity graph of Cu doped α-MnO2 (CMO
23
300 and CMO 600) is almost frequency independent and shows Plato region. This is because at low-
24
frequency region grain boundary behaves as poorly conducting barrier due to the formation of 16 ACS Paragon Plus Environment
Page 17 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
microstructural defects which restricts hopping frequency between free charge carriers. All the samples
2
were found to have a tendency to increase ac conductivity towards high-frequency. Ac electrical
3
conductivity is significantly high in case of CMO 600. This is because above 104 Hz hopping
4
frequency of free charge carriers within MnO2 crystal structure increases, which results in enhancement
5
of ac conductivity toward the high-frequency region.
17 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 18 of 36
1 2
Figure 6. Dielectric Analysis And Nyquist Plot: Variation of (a) Dielectric constant (εr) and its rescale
3
portion (Inset graph), (b) AC conductivity (σac) and (c) Tangent loss (tanδ) with frequency for the
4
sample CMO 300, CMO 450 and CMO 600. Variation of imaginary part of impedance (ZIM) with real
5
part of impedance (ZR) for (d) CMO 300 (e) CMO 450 and (f) CMO 600. 18 ACS Paragon Plus Environment
Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
Dielectric loss tangent (tanδ) (which is proportional to the loss of energy due to the applied electric
2
field into the sample) was calculated using the relation ε′′ = tanδ
3
(10)
4
Nature of frequency versus loss tangent plot displayed in Fig. 6(c) is in good agreement with the
5
finding of the dielectric plot. Dielectric behaviour of our sample follows Maxwell-Wagner-Sillars
6
interfacial type of polarization33. The nanocrystalline material consists of highly resistive grain
7
boundary which plays a crucial role in interfacial polarization process33. Due to Cu doping within
8
MnO2 structure, free charge career is released. Both Cu doping and calcination procedure introduce
9
some structural inhomogeneities like oxygen vacancy defects, dandling bond within MnO2 lattice
10
structure. These defect zones trap free mobile carriers during their migration through grain boundaries.
11
Trapped carriers then induce nano dipole and cause polarization when an external field is applied. This
12
interfacial space charge polarization contributes to the high value of dielectric constant near low-
13
frequency region52. When proceeding to the high-frequency region, induced nano dipole cannot follow
14
external field frequency variation which results in rapid fall of dielectric value and makes it
15
independent of external field frequency53.
16
To explain the contribution of both grain and grain boundaries towards dielectric constants
17
more precisely, another approach has been made through Nyquist plot analysis, shown in Fig.6.
18
Majority of the highly conducting grains are separated by highly resistive grain boundaries. In case of
19
sample annihilated at 300°C [CMO 300, Fig. 6(d)], the contribution of grain to the conductivity is high
20
since boundaries are highly resistive consisting of defect states. But when approach to higher sintering
21
temperature [say for sample CMO 450 (Fig. 6(e)) and CMO 600 (Fig. 6(f))] actually coalescence of
22
grain occurred and grain boundaries reduced defect states. So electron hopping through grain increased
23
and its potential barrier at boundary gets lowered that makes electron migrations easier33.
19 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2
Page 20 of 36
The observed variation of dielectric data with temperature ranging from room temperature up to 180°C has been presented in Fig 7(a).
3 4
Figure 7. Temperature Dependent Electrical Properties and Activation Energy: Variation of (a)
5
Dielectric constant (εr), (b) Tangent loss (tanδ) and (c) AC Conductivity (σac) with temperature at
6
different frequencies ranging from 40 Hz to 1 MHz for sample CMO 450 and (d) Value of activation
7
energies calculated using Arrhenius plot for the sample CMO 450.
8
It is clear from the graph that a ferroelectric phase transition temperature exists near at 66°C. Thereafter
9
the dielectric constant gradually decreases towards higher temperature. This actually happened since
10
thermally agitated dipoles due to a high rate of vibration get disoriented and decreases polarization
11
effect. Variation of tangent loss data with temperature has been displayed in Fig 7(b). It is evident from
12
the graph that tangent loss has significantly enhanced towards higher temperature. This is due to fact
13
that thermally agitated dipoles during their high rate of vibration cause absorption of microwave energy 20 ACS Paragon Plus Environment
Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
on account of inter-dipolar friction. Enhanced ac conductivity as shown Fig. 7(c) due to predominated
2
thermally activated type conduction mechanism is mediated by released free charge carrier from defect
3
species within the material. Temperature dependence of conductivity for sample CMO 450 has been
4
explained by Arrhenius plot (ln σ vs
5
higher temperature region is due to dominated thermally activated type conduction mechanism.
6
Activation energy has been estimated at some selected frequency (40Hz, 100Hz, 1KHz, 10KHz,
7
100KHz, 1MHz ) of applied field and were found to be 0.636eV, 0.620eV, 0.587eV, 0.572eV, 0.496eV
8
and 0.391eV respectively. These findings towards the higher temperature are in good agreement with
9
the enhanced ac conductivity data.
1 𝜏
) in Fig. 7(d). In which linear straight line fit with the curve at
10
Photoluminescence (PL) spectroscopy is an excellent technique to investigate the optoelectronic
11
properties of QD semiconducting material. The synthesis route of nanoparticles, their stoichiometry,
12
post-sintering and doping percentage in host material - all these are very sensitive to create various
13
surface morphologies and an introduction of microstructural defects within the nanocrystalline
14
materials54. PL intensity is directly correlated with all these defects formed34 inside the sample.
15
Photoluminescence spectra of entire samples have been recorded at room temperature under the
16
excitation wavelength (λexc) of 276 nm, as displayed in Fig. 8(a).
21 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 22 of 36
1 2
Figure 8. Light emission With Schematic Diagram: (a) 3D view of photoluminescence spectra for
3
entire samples. The samples are excited with excitation wavelength 276 nm. (b) Photoluminescence
4
emission spectra mediated by phonon vibration.
5
The room temperature PL shows some intense peaks both in the ultraviolet and in the visible region
6
which can be interpreted as due to radiative and non-radiative exciton recombination mediated by
7
defects and doping impurities15. The PL spectrum exhibits UV emission band near 332 nm and 380 nm
8
corresponds to the band edge emission signals due to recombination among free excitons through
9
exciton-exciton
collision
process
within
the
well-crystallized
MnO2 crystals54,39.
The
10
trapping/localization of charge carriers at the defect states results in quenching of the exciton emission.
11
Trapping leads to strong carrier localization, which decreases the overlap between the electron and the
12
hole wave function. The blue emission near 430 nm and 459 nm may be ascribed to the oxygen
13
vacancy-related defects39,54. A broad green emission was also observed at around 490 nm which can be
14
ascribed to the surface defects or surface dangling bonds39. The schematic diagram of band structure
15
has been shown in Fig. 8(b). Observed shift in the position and width of PL spectrum has occurred due 22 ACS Paragon Plus Environment
Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
to the variation of fundamental band-gap energy and strain developed in the sample caused by
2
combining effect of Cu doping and sintering process.
3
The red shifting of PL emission spectra is one of the well known consequence of quantum
4
confinement effect15,55 normally occurred for the particles nano regime and is proportional to the
5
reduction in exciton confinement and is larger for smaller offset (energy offsets among the three
6
materials are finite and therefore the exciton wave function partially extends into the material i.e,
7
‘exciton leakage’)15. The energy levels of semiconductor quantum dots depend on the particle-size
8
distribution which affects the PL mechanism (shown in supporting information). PL intensity of entire
9
CMO samples increases with the increase of sintering temperature. Most prominent blue emission peak
10
has been analysed and found that its intensity increased upto 24.58% for CMO 450 and 30.32% for
11
CMO 600 with respect to that of CMO 300. Quantitative analysis of PL intensity reveals that the
12
intensity, as well as the crystallinity of the materials, increases with the higher sintering temperature.
13
Efficient charge transport from traps or defect states to the emission centers and subsequent reduction
14
in non-radiative recombination centers at various sintering temperature may the cause of intensity
15
variation in PL56. So this material can be beneficiated for the fabrication of the light emitting device.
16
For the tuning capability as well as remarkable green emission zone in PL spectra can be utilized for
17
the construction of optical devices. Aforesaid defects also contribute to obtaining high dielectric
18
constant of our material which has been explained more precisely in the remaining part of the
19
preceding paper.
20 21
Fabrication of asymmetric supercapacitor:
22
In the two electrode hybrid asymmetric capacitor system, we have used thin copper foil as anode and
23
aluminium foil as cathode electrode. Both electrodes are coated with a thin layer of carbon black. This
24
active layer acts as energy storage material of the device. Polyvinyl alcohol (PVA)/CMO 450 solid thin 23 ACS Paragon Plus Environment
Langmuir
film was treated as high dielectric separator, used in between the two electrodes and sandwiched them properly. This fabricated device can able to illuminate 15 LEDs (each of 1V) for 47 min 23 sec (see the supporting document). The detail specifications and performance of our fabricated device is shown in figure 9 and table 2.
Figure 9. Device fabrication and performance: (a) Schematic diagram of series connected 3 devices and their charge transfer mechanism (b) Attempt to glow 15 LED by our fabricated device (c) dimension of our fabricated device (d) Thickness of the device (e) Flexiblity test (f) Quick charging ability confirmation (g) Discharging current profile (h) Discharging voltage profile. Table 2. Device specification and performance details Descriptions
1 2 1 3 4 2 5 6 3 7 8 4 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 5 36 37 6 38 39 40 7 41 42 8 43 44 9 45 46 47 10 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 24 of 36
Anode details
Material type
Copper
Dimension
1 cm × 1 cm
Thickness
100 um
24 ACS Paragon Plus Environment
Page 25 of 36
Cathode details
Specifications
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 1 29 30 2 31 32 3 33 34 35 4 36 37 5 38 39 6 40 41 42 7 43 44 8 45 46 9 47 48 49 10 50 51 11 52 53 12 54 55 56 57 58 59 60
Langmuir
Material type
Aluminium
Dimension
1 cm × 1 cm
Thickness
80 um
Active layer thickness
50um
Separator thickness
60um
No. of connected devices
3 in series connection
Load used
15 LEDs of 1volt
Charged upto
30 Seconds
Discharged upto
1100 Seconds
Discharging parameter
Current- 1.2 mA to 0.64 mA Voltage- 2.64 V to 2.28 V
Electrochemical performance: The synthesized high dielectric material (CMO 450) has been used as a separator of the fabricated device. The cyclic properties of a single device have been investigated using two electrode configurations by the help of a CHI 608E potentiostat-galvanostat instrument. The cyclic voltammetry (CV) measurement has been carried out in the potential range 0 to 0.9 V and shown in Fig. 10(a). A prominent pseudo capacitance nature with redox peak has been observed in the CV curve when the performance is registered at scan rate 100 mV/s. Galvanostatic charging discharging (GCD) experiment of the same device has been carried out in the potential range 0 to 0.9 V with the current density 1 A/g and shown in Fig. 10(b). From the GCD curve, the value of specific capacitance of the samples has been estimated using the equation: iΔ𝑡
Csp =mΔ𝑉
25 ACS Paragon Plus Environment
(11)
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 26 of 36
1
Where, i designates discharging current, Δt discharging time, ΔV the potential window of discharge
2
and m the mass of active material. The calculated value of Csp for a single device at constant current
3
density 1 A/g is 15.4 F/g. From the calculated specific capacitance value (Csp) of the device from CD
4
curve and the observed potential window (ΔV) from CV curve, the specific energy density (Esp) can be
5
estimated by the equation 1
Esp = 2×Csp × ΔV2
6
(12)
7 8
Again from the obtained specific energy density value, the specific power density of the material can
9
be calculated using the formula Psp= Esp/ Δt. The energy and power density at constant current 1 A/g
10
are 1.77 Wh Kg-1 and 57 W Kg-1 respectively. All these calculated parameters are very high for a 1cm
11
× 1cm single device. Besides high delivering power density, cycling stability is also a most important
12
parameter of a supercapacitor for their practical usage. Cyclic lifetime of the single device has been
13
tasted at current density 1 A/g within the same potential range. The device exhibited excellent cyclic
14
performance with 83% retention of its initial capacitance after completion of 1000 cycles.
15 16
Figure 10. Electrochemical performance: (a) CV curve at 100 mV/s scan rate, (b) Galvanostatic charge-
17
dischrge property and (c) Retention capacity of a single device.
18 19
CONCLUSIONS
26 ACS Paragon Plus Environment
Page 27 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
The size controlled α-MnO2 SQDs have been successfully developed by a modified chemical
2
synthesis technique at various high temperatures. From the investigation of performance modulation,
3
an optimized sintering temperature has been found where all the physical properties of SQDs are
4
maximized leading to the remarkable dielectric and optical properties. The diameter of Cu-doped α-
5
MnO2 SQD sintered at 450°C is usually ~7 nm which has been confirmed from HRTEM. On the basis
6
of the UV-visible absorption spectrum, the optical band gap energy of the α-MnO2 SQD is found to be
7
1.38 eV. Dielectric measurement showed that, little loading of copper leading to the higher dielectric
8
constant accompanied with low dielectric loss and higher ac conductivity, which is highly desirable for
9
high charge storage supercapacitor. All these results suggest that the SQDs containing Mn and Cu are
10
the promising candidates for high performance electronic and photovoltaic device applications. Due to
11
the simplicity to synthesize SQDs with tuneable sizes and various applicability, this approach may be
12
extended to synthesis other SQDs.
13 14
SUPPORTING INFORMATION
15
As the supporting information, a video (duration= 47 min 23 sec) of glowing 15 commercial Light
16
Emitting Diode (LED) is provided along with the manuscript.
17 18
ACKNOWLEDGMENTS
19
The authors gratefully acknowledge Dr. Achintya Singha of Bose Institute, Kolkata for Raman
20
spectroscopy measurement and also thankful to CIRE, Jadavpur for providing financial assistance and
21
IIT, Kharagpur for their instrumental facility.
22 23
REFERENCES
24
(1)
Moreels, I.; Lambert, K.; Smeets, D.; Muynck, D. De; Nollet, T.; Martins, J. C.; Vanhaecke, F.; 27 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Vantomme, A.; Delerue, C.; Allan, G.; et al. Size-Dependent Optical Properties of Colloidal
2
{PbS} Quantum Dots. ACS Nano 2009, 3 (10), 3023–3030.
3
(2)
Page 28 of 36
Chukwuocha, E. O.; Onyeaju, M. C.; Harry, T. S. T. Theoretical Studies on the Effect of
4
Confinement on Quantum Dots Using the Brus Equation. World J. Condens. Matter Phys. 2012,
5
2 (2), 96–100.
6
(3)
Surface Chemistry Study. ACS Nano 2012, 5 (3), 2004–2012.
7 8
(4)
Hoogland, S.; Sukhovatkin, V.; Howard, I.; Cauchi, S.; Levina, L.; Sargent, E. H. A SolutionProcessed 1.53 Μm Quantum Dot Laser with Temperature-Invariant Emission Wavelength. Opt.
9
Express 2006, 14 (8), 3273.
10 11
Moreels, I.; Justo, Y.; Geyter, B. De; Haustraete, K.; Martins, J. C.; Hens, Z. Quantum Dots: A
(5)
Greben, M.; Fucikova, A.; Valenta, J.; Greben, M.; Fucikova, A.; Valenta, J. Photoluminescence
12
Quantum Yield of PbS Nanocrystals in Colloidal Suspensions Photoluminescence Quantum
13
Yield of PbS Nanocrystals in Colloidal Suspensions. 2015, 144306 (May), 1–7.
14
(6)
Mcdonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E.
15
H. Solution-Processed PbS Quantum Dot Infrared Photodetectors and Photovoltaics. Nat. Mater.
16
2005, 4 (2), 138–142.
17
(7)
Molinari, E.; Nazionaleper, I. Information Processing in Quantum Dots. 2, 36–37.
18
(8)
Zhang, Y. Q.; Cao, X. A. Optical Characterization of CdSe Quantum Dots with Metal Chalcogenide Ligands in Solutions and Solids. Appl. Phys. Lett. 2011, 99 (2), 1–4.
19 20
(9)
325–331.
21 22
Sargent, E. H. Infrared Photovoltaics Made by Solution Processing. Nat. Photonics 2009, 3 (6),
(10)
Zhang, G.; Teng, F.; Zhao, C.; Chen, L.; Zhang, P.; Wang, Y.; Gong, C.; Zhang, Z.; Xie, E.
23
Enhanced Photocatalytic Activity of TiO2/carbon@TiO2 Core–shell Nanocomposite Prepared
24
by Two-Step Hydrothermal Method. Appl. Surf. Sci. 2014, 311, 384–390. 28 ACS Paragon Plus Environment
Page 29 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
(11)
Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Surface Plasmon Subwavelength Optics. Nature 2003, 424 (6950), 824–830.
2 3
Langmuir
(12)
Lehtimäki, M.; Hoffmannová, H.; Boytsova, O.; Bastl, Z.; Busch, M.; Halck, N. B.; Rossmeisl,
4
J.; Krtil, P. Targeted Design of ??-MnO2 Based Catalysts for Oxygen Reduction. Electrochim.
5
Acta 2016, 191 (January), 452–461.
6
(13)
Truong, T. T.; Liu, Y.; Ren, Y.; Trahey, L.; Sun, Y. Morphological and Crystalline Evolution of
7
Nanostructured MNO 2 and Its Application in Lithium-Air Batteries. ACS Nano 2012, 6 (9),
8
8067–8077.
9
(14)
Kim, S.; Lim, Y. T.; Soltesz, E. G.; De Grand, A. M.; Lee, J.; Nakayama, A.; Parker, J. A.;
10
Mihaljevic, T.; Laurence, R. G.; Dor, D. M.; et al. Near-Infrared Fluorescent Type II Quantum
11
Dots for Sentinel Lymph Node Mapping. Nat. Biotechnol. 2004, 22 (1), 93–97.
12
(15)
2011, 40 (3), 1512–1546.
13 14
Donegá, C. de M. Synthesis and Properties of Colloidal Heteronanocrystals. Chem. Soc. Rev.
(16)
Li, B.; Gao, G.; Zhai, D.; Wei, C.; He, Y.; Du, H.; Kang, F. Synthesis, Characterization and
15
Electrochemical Performance of Manganese Dioxide in a Quaternary Microemulsion: The Role
16
of the Co-Surfactant and Water. Int. J. Electrochem. Sci. 2013, 8 (6), 8740–8751.
17
(17)
Fan, R.-J.; Chen, Y.; Chen, B.-Z.; Wang, J.-W. Facile Synthesis and Electrochemical Properties
18
of α-MnO2 as Electrode Material for Supercapacitors. Asia-Pacific Journal of Chemical
19
Engineering. 2013, pp 721–729.
20
(18)
High Performance Redox Supercapacitors. Electrochem. commun. 2004, 6 (10), 1004–1008.
21 22
(19)
Li, Y.; Xie, H.; Wang, J.; Chen, L. Preparation and Electrochemical Performances of ??-MnO2 Nanorod for Supercapacitor. Mater. Lett. 2011, 65 (2), 403–405.
23 24
Rajendra Prasad, K.; Miura, N. Electrochemically Synthesized MnO 2-Based Mixed Oxides for
(20)
Li, J.; Ren, Y.; Wang, S.; Ren, Z.; Yu, J. Transition Metal Doped MnO2nanosheets Grown on 29 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 30 of 36
1
Internal Surface of Macroporous Carbon for Supercapacitors and Oxygen Reduction Reaction
2
Electrocatalysts. Appl. Mater. Today 2016, 3, 63–72.
3
(21)
You, B.; Li, N.; Zhu, H.; Zhu, X.; Yang, J. Graphene Oxide-Dispersed Pristine CNTs Support
4
for MnO2 Nanorods as High Performance Supercapacitor Electrodes. ChemSusChem 2013, 6
5
(3), 474–480.
6
(22)
Wang, Y.; Zhong, H.; Hu, L.; Yan, N.; Hu, H.; Chen, Q. Manganese hexacyanoferrate/MnO2
7
Composite Nanostructures as a Cathode Material for Supercapacitors. J. Mater. Chem. A 2013, 1
8
(7), 2621.
9
(23)
Modulation of α-MnO₂ Nanowires by Crystal Facet Engineering. Sci. Rep. 2015, 5, 8987.
10 11
Li, W.; Cui, X.; Zeng, R.; Du, G.; Sun, Z.; Zheng, R.; Ringer, S. P.; Dou, S. X. Performance
(24)
Zhu, J.; He, J. Facile Synthesis of Graphene-Wrapped Honeycomb MnO
2
12
Nanospheres and Their Application in Supercapacitors. ACS Appl. Mater. Interfaces 2012, 4 (3),
13
1770–1776.
14
(25)
Li, H.; Wang, W. Lou; Pan, F.; Xin, X.; Chang, Q.; Liu, X. Synthesis of Single-Crystalline α-
15
MnO2 Nanotubes and Structural Characterization by HRTEM. Mater. Sci. Eng. B Solid-State
16
Mater. Adv. Technol. 2011, 176 (14), 1054–1057.
17
(26)
Li, X.; Liu, J.; Zhao, Y.; Zhang, H.; Du, F.; Lin, C.; Zhao, T.; Sun, Y. Significance of Surface
18
Trivalent Manganese in the Electrocatalytic Activity of Water Oxidation in Undoped and Doped
19
MnO2 Nanowires. ChemCatChem 2015, 7 (12), 1848–1856.
20
(27)
Gao, T.; Glerup, M.; Krumeich, F.; Nesper, R.; Fjellvâg, H.; Norby, P. Microstructures and
21
Spectroscopic Properties of Cryptomelane-Type Manganese Dioxide Nanofibers. J. Phys. Chem.
22
C 2008, 112 (34), 13134–13140.
23 24
(28)
Julien, C. M.; Massot, M.; Poinsignon, C. Lattice Vibrations of Manganese Oxides: Part I. Periodic Structures. Spectrochim. Acta - Part A Mol. Biomol. Spectrosc. 2004, 60 (3), 689–700. 30 ACS Paragon Plus Environment
Page 31 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
(29)
(30)
Stella, C.; Soundararajan, N.; Ramachandran, K. Magnetic Properties of α-MnO2 and Co-Doped α-MnO2 Nanowires. AIP Conf. Proc. 2014, 1591, 423–425.
4 5
Cockayne, E.; Li, L. First-Principles DFT+ U Studies of the Atomic, Electronic, and Magnetic Structure of Alpha-MnO2(Cryptomelane). Chem. Phys. Lett. 2012, 544, 53.
2 3
Langmuir
(31)
Das, S.; Das, S.; Roychowdhury, A.; Das, D.; Sutradhar, S. Effect of Gd Doping Concentration
6
and Sintering Temperature on Structural, Optical, Dielectric and Magnetic Properties of
7
Hydrothermally Synthesized ZnO Nanostructure. J. Alloys Compd. 2017, 708, 231–246.
8
(32)
Substrate. InTech 2012, 59–85.
9 10
(33)
Aparna, P. U.; Divya, N. K.; Pradyumnan, P. P. Structural and Dielectric Studies of Gd Doped ZnO Nanocrystals at Room Temperature. 2016, No. February, 79–88.
11 12
Alias, R. The Effects of Sintering Temperature Variations on Microstructure Changes of LTCC
(34)
Das, S.; Das, S.; Sutradhar, S. Effect of Gd 3+ and Al 3+ on Optical and Dielectric Properties of
13
ZnO Nanoparticle Prepared by Two-Step Hydrothermal Method. Ceram. Int. 2017, 43 (9),
14
6932–6941.
15
(35)
Oxide Nanoparticles. Mater. Today Proc. 2016, 3 (6), 2069–2076.
16 17
(36)
Gao, T.; Fjellvåg, H.; Norby, P. A Comparison Study on Raman Scattering Properties of ??and ??-MnO2. Anal. Chim. Acta 2009, 648 (2), 235–239.
18 19
Deotale, A. J.; Nandedkar, R. V. Correlation between Particle Size, Strain and Band Gap of Iron
(37)
Hashem, A. M.; Abdel-Ghany, A. E.; El-Tawil, R.; Bhaskar, A.; Hunzinger, B.; Ehrenberg, H.;
20
Mauger, A.; Julien, C. M. Urchin-like α-MnO2 Formed by Nanoneedles for High-Performance
21
Lithium Batteries. Ionics (Kiel). 2016, 22 (12), 2263–2271.
22
(38)
Hsu, Y.-K.; Chen, Y.-C.; Lin, Y.-G.; Chen, L.-C.; Chen, K.-H. Reversible Phase Transformation
23
of MnO 2 Nanosheets in an Electrochemical Capacitor Investigated by in situRaman
24
Spectroscopy. Chem. Commun. 2011, 47 (4), 1252–1254. 31 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
(39)
Page 32 of 36
Toufiq, A. M.; Wang, F.; Ullah Shah, H. Synthesis and Characterization of MnO2 Nanowires:
2
Lattice Vibrations and Photoluminescence Properties. Phys. Status Solidi Curr. Top. Solid State
3
Phys. 2017, 14 (10), 2–4.
4
(40)
2016, 6 (1), 20539.
5 6
Gao, Y.; Zhao, X.; Yin, P.; Gao, F. Size-Dependent Raman Shifts for Nanocrystals. Sci. Rep.
(41)
Li, F.; Li, G.; Chen, H.; Jia, J. Q.; Dong, F.; Hu, Y. B.; Shang, Z. G.; Zhang, Y. X. Morphology
7
and Crystallinity-Controlled Synthesis of Manganese Cobalt Oxide/manganese Dioxides
8
Hierarchical Nanostructures for High-Performance Supercapacitors. J. Power Sources 2015,
9
296, 86–91.
10
(42)
of Gold Nanoparticles from UV-Vis Spectra. Anal. Chem. 2007, 79 (11), 4215–4221.
11 12
Haiss, W.; Thanh, N. T. K.; Aveyard, J.; Fernig, D. G. Determination of Size and Concentration
(43)
Thomas, S.; Nair, S. K.; Muhammad, E.; Jamal, A.; Al-Harthi, S. H.; Varma, M. R.;
13
Anantharaman, M. R. Size-Dependent Surface Plasmon Resonance in Silver Silica
14
Nanocomposites. Nanotechnology 2008, 19 (75710), 7.
15
(44)
Lin, K.-F.; Cheng, H.-M.; Hsu, H.-C.; Lin, L.-J.; Hsieh, W.-F. Band Gap Variation of Size-
16
Controlled ZnO Quantum Dots Synthesized by Sol–gel Method. Chem. Phys. Lett. 2005, 409 (4–
17
6), 208–211.
18
(45)
Rajamanickam, N.; Ganesan, P.; Rajashabala, S.; Ramachandran, K. Structural and Optical
19
Properties of ??-MnO2 Nanowires and ??- MnO2 Nanorods. AIP Conf. Proc. 2014, 1591 (July
20
2015), 267–269.
21
(46)
Aparna, S.; Elakhya, N.; Gopal, G.; Rajesh, P.; Ramasamy, P. Influence of Polyaniline in
22
Polyaniline-Tin Oxide Nanocomposite as Counter Electrode for Dye Sensitized Solar Cells.
23
Optik (Stuttg). 2018, 157, 1219–1226.
24
(47)
Alivisatos, A. P.; Alivisatos, A. P. Perspectives on the Physical Chemistry of Semiconductor 32 ACS Paragon Plus Environment
Page 33 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
Nanocrystals Perspectives on the Physical Chemistry of Semiconductor Nanocrystals. 1996, 100
2
(August), 13226–13239.
3
(48)
Bashir, B.; Shaheen, W.; Asghar, M.; Warsi, M. F.; Khan, M. A.; Haider, S.; Shakir, I.; Shahid,
4
M. Copper Doped Manganese Ferrites Nanoparticles Anchored on Graphene Nano-Sheets for
5
High Performance Energy Storage Applications. J. Alloys Compd. 2017, 695, 881–887.
6
(49)
Electrical Characteristics of α-MnO2 Nanorods. Powder Technology 2012, 224, 356–359.
7 8
(50)
(51)
Chen, C.; Qiu, X.; Ji, S.; Jia, C.; Ye, C. The Synthesis of Monodispersed AgBiS2 Quantum Dots with a Giant Dielectric Constant. CrystEngComm 2013, 15 (38), 7644.
11 12
Kumar, A.; Biradar, A. M. Effect of Cadmium Telluride Quantum Dots on the Dielectric and Electro-Optical Properties of Ferroelectric Liquid Crystals. Physical Review E 2011, 83 (4).
9 10
Guan, H.; Wang, Y.; Chen, G.; Zhu, J. Frequency and Temperature Effects on Dielectric and
(52)
Paul, B. K.; Roy, D.; Batabyal, S.; Bhattacharya, A.; Nandy, P.; Das, S. A Comparative Study of
13
Strontium and Titanium Doped Mullite in PVDF Matrix and Their Phase Behavior,
14
Microstructure and Electrical Properties. Mater. Chem. Phys. 2017, 187, 119–132.
15
(53)
Paul, B. K.; Halder, K.; Roy, D.; Bagchi, B.; Bhattacharya, A.; Das, S. Dielectric Switching
16
above a Critical Frequency Occured in Iron Mullite Composites Used as an Electronic Substrate.
17
J. Mater. Sci. Mater. Electron. 2014, 25 (12), 5218–5225.
18
(54)
Das, S.; Das, S.; Das, D.; Sutradhar, S. Tailoring of Room Temperature Ferromagnetism and
19
Electrical Properties in ZnO by Co (3d) and Gd (4f) Element Co-Doping. J. Alloys Compd.
20
2017, 691, 739–749.
21
(55)
Christopher, J. W.; Goldberg, B. B.; Swan, A. K. Long Tailed Trions in Monolayer MoS2:
22
Temperature Dependent Asymmetry and Resulting Red-Shift of Trion Photoluminescence
23
Spectra. Sci. Rep. 2017, 7 (1), 1–29.
24
(56)
Mu, Y.; Qiu, M.; Zhou, X.; Li, Q.; Sun, M.; Yang, H.; Fu, W.; Yang, L.; Ma, J. Effect of 33 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 34 of 36
1
Sintering Temperature on the Luminescence Properties of Ca 0.8 Ba 0.2 TiO 3 :Pr 3+ Red-Emitting
2
Phosphor. Luminescence 2015, 30 (5), 503–506.
3 4
Figure captions
5
Figure 1. Phase and Structural Model: (a) XRD pattern of CMO 300, CMO 450 and CMO 600 (b)
6
rescale portion of (211) peak of the entire sample (c) Characteristics Raman vibration mode of the said
7
samples (d) Proposed model of Cu-doped α-MnO2.
8
Figure 2. Microstructure and Quantitative Elemental Analysis: FESEM microstructure image of (a)
9
CMO 300, (b) CMO 450 and (c) CMO 600 and (d) EDX spectra of the as-prepared sample.
10
Figure 3. QD overview: (a) HR-TEM image of CMO 450 QD and corresponding size histogram (inset).
11
The Gaussian distribution shows the range of particle diameter around 7 nm (b) Fringe pattern at 2 nm
12
scale range shows well crystalline nature and measured interplanar separation at (200) plane is 0.14 nm
13
(Inset). HR-TEM images with size histogram of (c) CMO 300 and (d) CMO 600. (e) EDX spectra of as
14
synthesized sample.
15 16
Figure 4. Light Absorbance And Band Gap Properties: (a) UV-Vis absorption spectra of α-MnO2 QDs
17
(CMO 300, CMO 450 and CMO 600). Indirect band gap energies are plotted using Tauc plot (Inset
18
graph) and (b) Energy band structure with discrete electronic energy levels due to the variation of QD
19
particle size.
20
Figure 5. Semiconducting property analysis: Non-ohmic behavior of current (I)-voltage (V)
21
characteristics of the samples (a) CMO 300, (b) CMO 450 and (c) CMO 600.
34 ACS Paragon Plus Environment
Page 35 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
Figure 6. Dielectric Analysis And Nyquist Plot: Variation of (a) Dielectric constant (εr) and its rescale
2
portion (Inset graph), (b) AC conductivity (σac) and (c) Tangent loss (tanδ) with frequency for the
3
sample CMO 300, CMO 450 and CMO 600. Variation of imaginary part of impedance (ZIM) with real
4
part of impedance (ZR) for (d) CMO 300 (e) CMO 450 and (f) CMO 600.
5
Figure 7. Temperature Dependent Electrical Properties and Activation Energy: Variation of (a)
6
Dielectric constant (εr), (b) Tangent loss (tanδ) and (c) AC Conductivity (σac) with temperature at
7
different frequencies ranging from 40 Hz to 1 MHz for sample CMO 450 and (d) Value of activation
8
energies calculated using Arrhenius plot for the sample CMO 450.
9
Figure 8. Light emission With Schematic Diagram: (a) 3D view of photoluminescence spectra for
10
entire samples. The samples are excited with excitation wavelength 276 nm. (b) Photoluminescence
11
emission spectra mediated by phonon vibration.
12
Figure 9. Device fabrication and performance: (a) Schematic diagram of series connected 3 devices
13
and charge transfer mechanism (b) Attempt to glow 15 LED by our fabricated device (c) dimension of
14
our fabricated device (d) Thickness of the device (e) Flexiblity test (f) Quick charging ability
15
confirmation (g) Discharging current profile (h) Discharging voltage profile
16
Figure 10. Electrochemical performance: (a) CV curve at 100 mV/s scan rate, (b) Galvanostatic charge-
17
dischrge property and (c) Retention capacity of a single device.
18 19 20 21
Table captions Table 1. Sample Specifications: Structural parameters i.e., Lattice parameters (a, b, c), Crystallite size
22
(D), Unit cell volume (V), Microstrain (ε) and Dislocation density (δ)
23
Table 2. Device specification and performance details
24 25 35 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
For Table of Contents Use Only
26
36 ACS Paragon Plus Environment
Page 36 of 36
