Morphological, Structural, and Charge Transfer Properties of F-Doped

Subscriber access provided by Olson Library | Northern Michigan University

Article

On the Morphological, Structural and Charge Transfer Properties of F-Doped ZnO: A Spectroscopic Investigation Gian Paolo Papari, Brigida Silvestri, Giuseppe Vitiello, Luca De Stefano, Ilaria Rea, Giuseppina Luciani, Antonio Aronne, and Antonello Andreone J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04821 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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 free 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 accessible to all readers and 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.

The Journal of Physical Chemistry C 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 22

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

The Journal of Physical Chemistry

On the Morphological, Structural and Charge Transfer Properties of F-Doped ZnO: a Spectroscopic Investigation Gian Paolo Paparia, Brigida Silvestrib, Giuseppe Vitiellob,c, Luca De Stefanod, Ilaria Rea d, Giuseppina Lucianib*, Antonio Aronneb and Antonello Andreonea a

Dipartimento di Fisica, Università di Napoli “Federico II”, and CNR-SPIN, UOS Napoli, via Cinthia, I-80126, Napoli, Italy

b

Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale (DICMaPI), Università di Napoli “Federico II”, Piazzale Tecchio 80, I-80125 Napoli, Italy.

c

CSGI, Consorzio interuniversitario per lo sviluppo dei Sistemi a Grande Interfase, Sesto Fiorentino, via della Lastruccia 3, Firenze, Italy. d

CNR-IMM, SS Napoli, Istituto per la Microelettronica e Microsistemi, Consiglio Nazionale delle Ricerche, Via P. Castellino 111, I-80131, Napoli, Italy

[email protected]

Abstract: Different spectroscopic techniques have been applied to fluorine doped ZnO powders prepared through hydrothermal synthesis, to discern the effective capability of F atoms to improve ZnO conductivity. From XRD analysis, no lattice distortion was observed up to F doping 5 at% concentration. Photoluminescence measurements and electron paramagnetic resonance data show that F atoms tend to occupy oxygen vacancies, inducing the onset of luminescence centers. The resulting doping effect consists into the increment of localized charge, as also proved via THz spectroscopy, where the Drude-Smith model has been applied to extract quantitative information on the electrodynamic parameters of ZnO:F samples. Results show that F doping does not produce any substantial change of plasma frequency but only the enhancement of scattering rate due to an increase of grain boundary density. Our measurements are in agreement with theoretical calculations asserting that the energy required to excite donor levels is of the order of 0.7 eV and therefore the doping mechanism is ineffective at room temperature.

ACS Paragon Plus Environment


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 22

Introduction The large bandgap (3.3 eV)1, visible light transparency, availability and multiple techniquesfor thin film deposition (chemical vapor deposition, sputtering, molecular beam epitaxy, etc.) make zinc oxide (ZnO) a versatile semiconductor extensively used in different technological applications2,3 (and references therein). Solar cells 4–10, piezoelectric actuators11,12 and thin film transistors are just a few examples of technological returns of ZnO. Different kind of doping have been applied to ZnO, aiming at improving conducting properties but preserving its transparency

8,9,13,14

.Successful

results have been reported in the attempt to dope ZnO with fluorine (F),since an additional electron with respect to oxygen is naively considered a “shallow donor”2,8,9,15. Nevertheless, the mechanism through which F atoms can provide free carriers is still controversial

2,16,17

. Indeed, some authors

predict an increase of extra delocalized electrons as well as overall carrier mobility due to F-doping 18, 19

. On the other hand, a decrease of conductivity of F-doped ZnO is expected by other studies

12

.

Indeed, fluorine atoms are supposed to occupy oxygen vacancies which are the most favorable defects in ZnO 2. However, F atoms are theoretically expected to widen the distance between neighboring Zn atoms, producing a deep localized state 2. Density functional theory has been employed to account for the effect of fluorine doping on ZnO:F transport properties. F atoms can work as either donor or acceptor according to the case they get full of oxygen vacancies or remain interstitials

16

. The resulting activation energies have been estimated to be too high (~0.7) for

effectively providing any kind of carrier at room temperature, and the increase of free charge density could be ascribed to the effect of surface passivation operated by fluorine. A number of morphologies of nanocrystalline ZnO, such as nanoparticles, nanowires, nanosheets, nanorods, nanotubes, nanoflowers have been successfully prepared by various synthetic methods 20 ,hydrothermal, sol–gel, direct precipitation being the most important ones

21,22

. Nowadays, the

hydrothermal method is considered the most convenient synthetic procedure because of its low cost, mild experimental conditions and environmental friendliness

23

. Indeed, the relevant chemical

reactions occur in aqueous solution, employing low temperatures and cheap reagents. Besides that, it shows a broad control on the chemical components, offering a high flexibility in terms of final composition as well as final nanostructure morphologies. Indeed, ZnO crystals exhibit different morphologies from wire, rod, tube, ring, spring to plate, comb, propeller, tetrapod as well as complex superstructures such as nanoscrews, nanodisks and their stacks are hugely influenced by the chemical environment employed for nucleation and crystal growth 24. In particular, F- ions, usually used as doping precursors, play a significant role in defining the morphology of ZnO nanostructures 20, 25. ACS Paragon Plus Environment

Page 3 of 22

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


Fruitful insights into the effect of fluorine doping in ZnO can be gained through spectroscopic methodologies. Among several probing experiments, Electron Paramagnetic Resonance (EPR) offers a unique way to deduce many structural and electronic information as well as defects in semiconductor species 26. Photoluminescence spectroscopy (PL) is an effective technique to study electronic band structure as well as defect states of semiconductor materials

27

. Furthermore,

Terahertz Time Domain Spectroscopy (THz-TDS) is a powerful technique for quantitatively studying the effect of doping on the change of complex permittivity ̃ = + of the material. Basically, the change in the real part releases information on the localized charge, whereas the imaginary part is linked to the presence of free charge.THz electrodynamics of ZnO thin films and nanostructures with different level of intrinsic conductivity already appeared in recent literature , however the issue of the change of ̃ as a function of the fluorine concentration has not been

28-32

addressed yet. In this work, ammonium hydrogen fluoride, NH4FHF, was used as fluorinating agent for the preparation of F-doped ZnO powders by hydrothermal synthesis, exploring the following F atomic concentration range: 0 at% (bare, sample name ZnO:F0), 1 at%(ZnO:F1), 3 at%(ZnO:F3), 5 at% (ZnO:F5). This precursor is an easy handled solid (Tf = 125 °C) whose dissociation gives not only F- but also H+ and HF2- ions with HF molecules 33. The work is aimed to elucidate the role played by the fluorine precursor in defining the morphological, electrical and optical properties of F-doped ZnO samples using different and complementary spectroscopic techniques such as THz-TDS, EPR and photoluminescence (PL) spectroscopy. Particularly, quantitative information on intrinsic ZnO:F conductivity can be obtained through THz-TDS. The Landau–Lifshitz–Looyenga mean field theory 34

has been applied to extract intrinsic information on ̃ of ZnO according to the various doping

level of fluorine. The different spectroscopy techniques have been also combined with Scanning Electron Microscopy (SEM), in order to correlate the morphology of ZnO and ZnO:F powders with the electronic and conducting properties, with the final aim to provide a rationale behind the controversial data on the electrical behavior of F-doped systems. Overall this study is expected to deliver strategic knowledge to the design of ZnO based electrical and sensor devices with tuned optoelectronic properties.



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

Materials and Methods Bare and F-doped ZnO powders were prepared by the hydrothermal route. Typically, a solution containing a 1:1 molar ratio of triethylamine (TEA) and zinc acetate di-hydrate (ZAD) was prepared, mixing TEA (0.84 mL) and ZAD (1.32 g) into ethanol (36.0mL) under constant stirring. After complete dissolution of ZAD, water (4.0mL) was added drop-wise, producing a whitish suspension. Afterwards, different amounts of ammonium hydrogen fluoride (NH4FHF) were used to realize the following atomic F concentrations: 0 at%, 1 at%, 3 at%, and 5 at%. The obtained suspensions were sealed within a Teflon recipient (the liquid volume corresponding to 75% of the whole), placed into a circulating oven and kept at 120 °C overnight. After cooling down to room temperature, powders were recuperated by centrifugation and repeatedly washed (3 times with distilled water). Prepared samples will be named thereafter F0, F1, F3, F5, respectively. Scanning Electron Microscopy was performed through a FESEM ULTRA- PLUS (Zeiss) at 20 kV with the SE2 detector and 15.9 mm working distance. The samples were gold-sputtered (3 nm thickness) with a HR208Cressington sputter coater. Electron Paramagnetic Resonance measurements were carried out by means of X-band (9 GHz) Bruker Elexys E-500 spectrometer (Bruker, Rheinstetten, Germany), equipped with a super-high sensitivity probe head. Solid samples were transferred to flame-sealed glass capillaries coaxially inserted in a standard 4 mm quartz sample tube. Measurements were performed at room temperature. The instrumental settings were as follows: sweep width, 100 G; resolution, 1024 points; modulation frequency, 100 kHz; modulation amplitude, 1.0 G. The amplitude of the field modulation was preventively checked to be low enough to avoid detectable signal over-modulation. EPR spectra were measured with a microwave power of ~0.6mW to avoid microwave saturation of resonance absorption curve. Few scans, typically 4, were accumulated to improve the signal-tonoise ratio. The g-factor was evaluated by means of an internal standard (Mg/MnO)35 which was inserted in the quartz sample tube co-axially with the capillary containing the samples. Steady-state PL spectra were recorded under excitation of a continuous light emission by a He–Cd laser at 325 nm (KIMMON Laser System, Japan). PL was collected at normal incidence, with respect to the surface of the samples, through an optical fiber coupled with a monochromator (Princeton Instruments, SpectraPro 300i), and detected by a Peltier cooled charge coupled device (CCD) camera (PIXIS 100F). A long pass filter, with a nominal cut-on wavelength of 350 nm, was used to remove the laser line at monochromator inlet.


Page 4 of 22

Page 5 of 22

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


THz

spectroscopy

(MenloSystems

TM

measurements

were

carried

out

using

a

Time-Domain

apparatus

). Generation and detection of THz signal is accomplished through the use of two

photoconductive antennas based on GaAs and excited by a laser pulse with 1500nm wavelength. THz signal is collimated and then focalized on the sample with a beam spot of about 2mm in diameter. Temporal sampling rate is fixed at about 0.1 ps, whereas the whole acquisition interval is about 200 ps. The free space signal has a bandwidth extension up to about 3 THz, roughly halved in the case of ZnO:F samples because of the high level of absorption they present. In order to optimize the information significance of the transmitted signal, some pellets, roughly 330µm thick, of ZnO:F/KBr in the ratio 1:10 were opportunely prepared. Pellets were obtained by pressing a mixture of 90% of KBr powder with the 10% of ZnO:F. KBr was chosen because sufficiently transparent up to about 2.5 THz, behaving therefore as an optimal host for THz spectroscopy. This procedure gives the advantage to precisely control within a few microns the thickness of each sample that is a crucial parameter for THz spectroscopy. In Fig.1 a sketch of the setup is reported. Pellets were mounted on a movable stage capable to measure in a single run the entire set of samples, the pure KBr pellet and the free space signal. The experiment was performed purging the measurement box with nitrogen gas and reaching a level of humidity lower than 0.1%.Data shown here are very robust because for each sample, the electric field signal vs time has been averaged over different runs.

Theoretical background Electrodynamic properties of pellets have been studied by applying a standard approach based on the computation of the complex transfer function ( = 2), experimentally obtained through

and the signal collected in free the ratio between the signal transmitted through the pellet

space . The modulus of transmission can be expressed as

= = ̃ ̃ ! "− $% + $%

*+ =

& '

( ∙ *+

.% 0.%

6

(1)

1

1 − -.%/4.%123 5 exp -− 2$% /

123

& '

5

where ̃ and ̃ are the Fresnel coefficients at the boundaries air-sample and sample-air

respectively,$% is the complex refractive index of the sample, $% the refractive index of air, d the

sample thickness and c is the speed of light. The factor FP accounts for the oscillations due to the



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 22

Fabry-Perot effect. A commercial software (Teralyzer) has been used to achieve $ : = $ + ; for each single pellet. This routine is based on the total variation technique

36–39

which defines the

refractive index $ , the extinction coefficient ; , and the effective optical thickness > , related to the minimum amplitude of FP oscillations. The effective optimal thickness is achieved with a precision of 2 µm, that reflects into an upper bound error (at 1 THz), for a single ̃ computation, of about ∆ ≅ ±7% for the real part and of about ∆ ≅ ±1% for the imaginary one. Since C =

D (see below), the uncertainty on conductivity is worth ΔC ≅ D Δ ≅ ±6%. The complex

dielectric function ̃ /D = + of each pellet is easily obtained through the relations = $6 − ;6 and = 2$ ; 40, with D as vacuum permittivity.

In order to extract the intrinsic value of the dielectric function of ZnO:F compounds, we employed the mean field theory (Landau–Lifshitz–Looyenga model 34), in analogy to previous experiments 41. Briefly, we used the following formula M/N N

M/N

ε%I ηI = Kε%L -1-ηI ∙ ε%P Q /ηI N (2) relating the guest substance (ZnO:F)permittivity ̃R to the permittivity of both the host substance ̃S

(KBr) and sample (mixture) substance ̃ (ZnO:F/KBr). The coefficients TR,S represent the concentrations in volume of guest and host substance and are related to the experimental concentrations in weightVR , through the equation: XY M0[Y

TR = W1 + X

Z

[Y

0M

\

(3)

With ]R and ]S mass density of the guest and host substance respectively, for which we assume the values ]R = ]^._ = 5.61a/bcN and ]S = ]de = 2.75 a/bcN .

Permittivity and conductivity of ZnO:F pellets have been fitted using a conventional Drude-Smith model 42,43. Complex permittivity can be written as i h

̃ = f − gi 4 -1 + bM j

j

j 0

5k

(4)

where f is the asymptotic value of , l6 = $ 6 /D c∗ is the plasma frequency, n and c∗ are the

carrier density and effective mass respectively, and n = 1/o is the relaxation frequency. The

coefficient bM ranges between -1 and 0 and accounts for the portion of localized/backscattered electrons. THz conductivity can be simply obtained through the expression C% = − p̃ − f qD ACS Paragon Plus Environment

(5)

Page 7 of 22

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


and from here the dc conductivityC&' , using C&' = 1 + bM l6 D /n

(6)

Results and discussion SEM analysis SEM micrographs of bare and F-doped ZnO powders, reported in Fig. 2, show a distribution of grains a few microns wide, displaying a truncated hexagonal pyramid shape because of the wurtzite crystal structure. At a closer look these microcrystals reveal an agglomerated structure and a particulate core, along the c axis, similar to “the stack of pancakes” typical of mesocrystals 24. The change of morphology induced by a few percent doping is impressive as shown in SEM images of ZnO:F1 and ZnO:F3, reported in Fig.2, where ZnO grains are surrounded by smaller ones, a few hundreds of nanometers wide (Fig. 2). In ZnO:F5, the density of small grains decreases, nevertheless the morphologic scenario of doped samples remains remarkably different from the ZnO case as shown (Fig.2). As shown by SEM micrographs at high magnification, both size and morphology of small nanoparticles are similar to those forming hexagonal crystals (Fig. 3). Actually, upon observation of the top and bottom surfaces of mesocrystals layers in ZnO:F3 and ZnO:F5 samples (Fig.3), we can conclude that small nanoparticles are leached from mesocrystals, leaving a weathered rough surface. XRD patterns of the samples (Fig.4a) confirm the formation of wurtzite ZnO structures (JCPDS 792205). Since the ionic radius of the F- anion (F-≅1.36Å) is similar to the O2- anion (O2-≅1.40Å), no lattice distortion can be appreciated. Furthermore, two peaks can be seen in an enlarged pattern at around 34° (Fig. 4b), probably due to the coexistence of different populations of particles with the same crystalline structure. Morphology evolution with the fluorine precursor content can be explained if we consider that both NH4+ and F- ions in solutions can interact with ZnO crystals. Actually, NH4+, F-and [Zn(NH3)4]2+ complex species in solution can diffuse to the surface of preformed ZnO crystals and be selectively adsorbed on specific crystallographic planes, eroding them and preventing the anchoring of new ions24. This effect can be also enhanced by the presence of other species, produced by the dissociation of NH4FHF such as H+, HF2- and HF. The adsorption processes of HF molecules, HF2- and H+ ions also modulate the dissolution rate 33 and influence the final crystallographic organization. Preferable dissolution occurs along the (002) crystallographic plane, as observed in SEM micrographs in Fig.2. Thus, if the concentration of NH4FHF increases in



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

the starting solution, a heavier chemical etch could take place, leading to a higher fraction of smaller particles (Fig. 2)25.

EPR analysis A micro-structural characterization of ZnO powders has been also realized by performing EPR spectroscopy44. This technique is a useful tool to monitor the presence of radical species45,46 and the behavior of the present native defects, such as oxygen and/or zinc vacancies. EPR spectra of the pristine and F-doped ZnO powders have been recorded, as shown in Fig. 5. For all samples, two signals at g = 1.9580 and g = 2.0024 have been observed. Although the attribution of the first peak is still highly controversial, it is usually ascribed to the shallow donor caused by the surface oxygen vacancies (VO) and interstitial Zn atoms (Zni)47. At the same time, the signal at g = 2.0024 is attributed to the presence of Zn vacancies (VZn)47,48. These experimental evidences confirm the presence of a high concentration of defects in the ZnO powders. Also, a sharp decrease in the intensity of peak at g = 1.9580 has been observed for the three ZnO:F samples with respect to the bare ZnO. This clearly indicates that the relative concentration of VO in ZnO:F is lower than in the bare sample, confirming that F atoms tend to occupy oxygen vacancies.

Photoluminescence spectroscopy Under laser irradiation, nanostructured ZnO normally emits a characteristic photoluminescence (PL) spectrum, which presents a very intense near-band-edge (NBE) ultraviolet peak at about 380 nm, due to free excitonic emission, which means transitions of free carriers from valence band to conduction band (VB->CB), and a broad peak in the visible-near infrared range related to material defects49,50.Valence-conduction band transitions and material defects contribution in the bares ample (pure ZnO) are clearly visible in Fig. 6(a), where the UV peak is centered at 386 nm and the visible broad peak has its maximum at 500nm. For the fluorine incorporated samples, the same components in the emission spectrum exist (see Fig. 6(b), (c), (d)) but slightly shifted toward lower frequencies, due to the increment in the optical band gap ascribed to Burnestein-Moss effect as a consequence of the F-doping19. The UV NBE emission of free excitons recombination is more energetic due to band distortions. More specific insight on the optical properties of the samples comes from deconvoluted PL spectra that are reported in Fig. 6(e) and (f). From Fig. 6(e), where ACS Paragon Plus Environment

Page 8 of 22

Page 9 of 22

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


deconvolution in Gaussian peaks of ZnO:F0 PL curve is shown, it is clear that the NBE peak is quite perfectly fitted by a single curve (R2= 0.996) related to VBZn->CBZn, whereas in some cases a second component could arise due to interstitial Zn atoms contribution (Zni->VBZn). Since this last contribution is absent, we can conclude that the contribution from Zn interstitial atoms is negligible 19

. The broad peaks in the visible range (≅ 499, 550 and 623 nm, R2= 0.998) are generally accepted

to be related to the radiative recombination of photo-generated holes with electrons occupying singly ionized VO’s, as well as to interstitial atoms of O (Oi) or –OH groups19. In particular, the peaks at 499 and 550 nm are ascribed to oxygen vacancies deep level emissions, where the difference is in the number of neighbors occupied by VO respectively all or one/two, and the peak component at 623 to Oi and surface –OH 51, 52. Deconvolution analysis of ZnO:F5 (Fig. 6(f)) reveals same spectrum features shifted by about10 nanometers (R2= 0.995 ≅ 381 nm, R2= 0.999 ≅ 492, 540 and 612 nm). PL characteristics strongly depend on F atoms percentage content. Fig. 7(a) reports the UV peak shift as a function of F atoms percentage. The band distortion is nearly proportional to F atoms, even if a saturation effect appears for the F5 sample. On the other hand, the intensities of these broad bands (Fig.7(b)) depend on the number of luminescence centers too. A higher concentration of VO is exhibited by the bare ZnO nanoparticles. For all the ZnO:F doped samples, the reduction in the intensity of visible emission is due to the decrease of VO number, since the F atoms effectively fill VO sites

48

. In particular, the intensity of the band at 623 nm,

associated to Oi and –OH surface group, decreases in ZnO:F samples, supporting the hypothesis that ionic species in solution adsorb onto ZnO surface, replacing –OH groups. Therefore, accordingly with EPR results, PL spectra confirm the F doping of ZnO sample, but at the same time the increase of doping precursor led to an increased fraction of smaller particles that was highest in the ZnO:F5 sample.

THz spectroscopy An expanded view on the measured time dependent THz signals is shown in Fig. 8(a), where the reference (free space), ZnO:F0/KBr, ZnO:F1/KBr, ZnO:F3/KBr and ZnO:F5/KBr sample curves are reported as solid lines with different colors. The average delay between the reference and the ZnO:F/KBr samples is mostly due to the contribution of the KBr which occupy the 90wt% of each pellet mixture. Instead, the effect of losses is more pronounced by comparing the transmission moduli rras reported in Fig. 8(b).The average transmission amplitude undergoes a robust

enhancement of about 50% passing fromZnO:F0 to ZnO:F1.Increasing the doping level,r rin doped samples decreases, remaining anyway more transparent than pure ZnO. All samples display ACS Paragon Plus Environment


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

significant Fabry-Perot oscillations, whose periodicity

∆ =

53

'

6.h &

Page 10 of 22

≈ 0.2 tu yieldsa refractive

index for a pellet $l ≈ 2.3,in agreement with the value achievable by temporal delay of THz signals $l = b

wx &

+ 1 for < ≅ 330 yc and z ≅ 1.4 !|.

Following the theoretical approach described in the previous section, we have extracted the intrinsic ̃ for all samples. In Fig.9 the complex dielectric function and the THz conductivity are reported in

panels (a) and (b) respectively. The real part of permittivity tends to increase passing from ≅ 10

for ZnO:F0 to ≅ 14 for ZnO:F5 at 1 THz. Instead, describes more complicate dynamics as the

doping increases. At 1 THz, the bare sample has ≅ 18 whereas for ZnO:F1 the value drops at

≅ 10.5, losing about 42% of its value. As the doping is further increased gets closer to the

value of bare ZnO, although ZnO:F5 still has ≅ 15. The samples photoconductivity can be easily gained through C = D , whose dependence on fluorine doping is clearly similar to the

dynamics, as reported in Fig.9(b). Taking as a reference the extracted direct conductivity, we can quantify the decrease of ZnO:F transport properties, noting that at 1 THz we find:C&' ~$: *1 ×

2.7 ≅ C&' ~$: *0 and C&' ~$: *5 × 1.6 ≅ C&' . Trends of ̃ components and C of different

ZnO:F powders are well defined against the intrinsic uncertainty represented through error bars, whose amplitude has been discussed in a previous section. THz and dc conductivity of the bare sample are in agreement with similar measurements performed on zinc oxide thin films

13, 32

. Instead, as plotted in Fig.9 (b), THz conductivity of ZnO:F1

undergoes a relevant decrease passing from about 10 S/cm, in the absence of doping, to about 6 S/cm. In order to find more quantitative information on the intrinsic conductivity we have fitted for all samples ̃ with the Drude-Smith model (eq. (4)). Fitting curves are reported as solid lines in Fig. 9(a) and (b) whereas the extracted parameters are summarized in Tab.1, which are affected by an uncertainty of about 2%. Since ZnO:F1 and ZnO:F3 show a very similar behavior, the fitting curve of ZnO:F3 has not been displayed for the sake of clarity. Within the Drude –Smith model, the conductivity decreases because of the contemporary increase of |bM |and the relaxation time n (eq. (6)). Indeed, both parameters increase by more than 20% in ZnO:F1 and ZnO:F3. In ZnO:F5, instead, this increase is of the order of 10% only, in spite of a relevant decrease of its conductivity in comparison with the bare sample. The lowering of conductivity can be better accounted for by looking at the change in morphology of ZnO:F powders, as shown by SEM analysis. According to the Drude-Smith model

42,54

, specifically

developed to account for the rate of backscattering processes in granular systems, the growth of |bM | is coherent with a system in which the number of grain boundaries involved into the relaxation ACS Paragon Plus Environment

Page 11 of 22

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


process increases. Since the relative variation of relaxation time n = 1/o follows the variation in |bM |, we can infer that the Mattheissen’s rule

1/o=1/o +1/oR holds. Here o accounts for the

54

relaxation process inside a single grain and oR = 1/R relates to the average scattering time at

grain boundaries. As fluorine doping is increased, the change in morphology suggests that oR

decreases, so that n basically follows the change in R . The consequences of an effective doping mechanism would reflect into a strong enhancement of l because of the growth of carrier density.

Instead, we observe a slight upshift of about 2%, hardily explainable in terms of an effective doping mechanism. On the other hand, the real part of permittivity tends to grow as a function of doping, and the reference value f in ZnO:F5, is about 20% larger than the bare value. This is the signature of some localized (polarizable) charge provided by the doping process. The enhancement of f is reasonably ascribable to the enhancement of luminescence centers observed in PL measurements too. We can therefore summarize the most significant results achieved using the THz spectroscopy in the following list, as a function of the doping increase: •

C&' first decreases and then increases again, however never restoring the value of the bare sample;

• • •

both |bM | and n reasonably increase for the rise of scattering rate at grain boundaries;

l remains basically constant; f tends to increase.

It is remarkable to note that the above Drude-Smith parameters keep their significance and trends within the highest uncertainty inherited from ̃. We infer therefore that the fluorine doping slightly affects the polarization charge of ZnO:F powders but does not enhance the carrier density. ZnO:F samples instead show a granular morphology at nanoscale, compatible with the growth of the grain boundary scattering rate and the decrease of C&' . The increase of the fraction of smaller particles with the amount of doping precursor

can

explain

this

effect.

Experimental

outcomes

are

in

agreement

with

photoluminescence experiments, as previously described and well supported by morphological evolution of ZnO structures in solution, with the increase of doping precursor concentration, which ultimately acts also as a leaching agent

24

. Our results on ZnO:F conductivity match theoretical

calculations reported in 16, where authors argue that fluorine doping cannot provide free carriers at room temperature. The enhancement in transport conductivity of fluorine doped ZnO thin films9, is a result difficult to compare, because those samples are largely different in fabrication ACS Paragon Plus Environment


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

processes, morphology and pristine conductive state. Indeed, bare films are composed by a compact joint of randomly distributed hexagonal thin layers and have a pristine conductivity on the order of C&' ≅ 17 /bc, that is more than double higher than our pristine powders.

Conclusions Fluorine doped ZnO powders have been prepared through hydrothermal synthesis and characterized via different spectroscopic investigations. Our results show that F atoms do not work as donors but rather affect the morphology of ZnO crystals only, promoting the onset of a granular phase. EPR and PL measurements proved the effective F doping of ZnO structures, showing that, increasing the doping level up to 5%, F atoms mainly occupy oxygen vacancies, inherently present in the bare ZnO. The increase of luminescence centers revealed via PL data well corresponds to the rise of asymptotic permittivity detected by THz spectroscopy. THz TDS measurements confirm that ZnO:F powders do not show an increment in free carrier charge. Indeed, the doping precursor exerts a relevant leaching on ZnO mesocrystals, leading to an increase of the fraction of smaller nanoparticles and, consequently of scattering rate at grain boundaries, that ultimately affects conductivity as confirmed by THz TDS spectroscopy. Our outcomes are in accordance with theoretical calculations asserting it is not possible to provide free carriers in ZnO through F doping at room temperature. Experimental verification of free charge enhancement in ZnO thin films can be reasonably related to a different role absolved by F atoms, which can work as passivation layer preserving the intrinsic conductivity of zinc oxide.

Acknowledgments The authors thank Dr. Manlio Colella, Centro di Ricerca Interdipartimentale sui Biomateriali, CRIB, Università di Napoli Federico II, for his technical assistance in SEM analyses.

References (1)

Mang, A.; Reimann, K.; Rübenacke, S. Band Gaps, Crystal-Field Splitting, Spin-Orbit Coupling, and Exciton Binding Energies in ZnO under Hydrostatic Pressure. Solid State Commun. 1995, 94 (4), 251–254.

(2)

Janotti, A.; Van de Walle, C. G. Fundamentals of Zinc Oxide as a Semiconductor. Rep. Prog. Phys. ACS Paragon Plus Environment

Page 12 of 22

Page 13 of 22

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


2009, 72 (12), 126501. (3)

Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys. 2005, 98 (4), 1– 103.

(4)

Azarang, M.; Shuhaimi, A.; Yousefi, R.; Sookhakian, M. Effects of Graphene Oxide Concentration on Optical Properties of ZnO/RGO Nanocomposites and Their Application to Photocurrent Generation. J. Appl. Phys. 2014, 116 (8).

(5)

Saáedi, A.; Yousefi, R.; Jamali-Sheini, F.; Zak, A. K.; Cheraghizade, M.; Mahmoudian, M. R.; Baghchesara, M. A.; Dezaki, A. S. XPS Studies and Photocurrent Applications of Alkali-MetalsDoped ZnO Nanoparticles under Visible Illumination Conditions. Phys. E Low-dimensional Syst. Nanostructures 2016, 79, 113–118.

(6)

Dezfuly, R. F.; Yousefi, R.; Jamali-Sheini, F. Photocurrent Applications of Zn(1−x)CdxO/rGO Nanocomposites. Ceram. Int. 2016, 42 (6), 7455–7461.

(7)

Kharatzadeh, A.; Jamali-Sheini, F.; Yousefi, R. Excellent Photocatalytic Performance of Zn(1−x)MgxO/rGO Nanocomposites under Natural Sunlight Irradiation and Their Photovoltaic and UV Detector Applications. Mater. Des. 2016, 107, 47–55.

(8)

Gordon, R. G. Deposition of Transparent Conducting Oxides for Solar Cells. AIP Conference Proceedings. 1997, pp 39–48.

(9)

Liang, H.; Gordon, R. G. Atmospheric Pressure Chemical Vapor Deposition of Transparent Conducting Films of Fluorine Doped Zinc Oxide and Their Application to Amorphous Silicon Solar Cells. J. Mater. Sci. 2007, 42 (15), 6388–6399.

(10)

Tari, O.; Aronne, A.; Addonizio, M. L.; Daliento, S.; Fanelli, E.; Pernice, P. Sol–gel Synthesis of ZnO Transparent and Conductive Films: A Critical Approach. Sol. Energy Mater. Sol. Cells 2012, 105, 179–186.

(11)

Li, T.; Li, Y. T.; Qin, W. W.; Zhang, P. P.; Chen, X. Q.; Hu, X. F.; Zhang, W. Piezoelectric Size Effects in a Zinc Oxide Micropillar. Nanoscale Res. Lett. 2015, 10 (1), 394.

(12)

Zhang, Y.; Liu, C.; Liu, J.; Xiong, J.; Liu, J.; Zhang, K.; Liu, Y.; Peng, M.; Yu, A.; Zhang, A.; et al. Lattice Strain Induced Remarkable Enhancement in Piezoelectric Performance of ZnO-Based Flexible Nanogenerators. ACS Appl. Mater. Interfaces 2016, 8 (2), 1381–1387.

(13)

Benramache, S.; Belahssen, O.; Temam, H. Ben. Effect of Band Gap Energy on the Electrical Conductivity in Doped ZnO Thin Film. J. Semicond. 2014, 35 (7), 73001.

(14)

Addonizio, M. L.; Aronne, A.; Daliento, S.; Tari, O.; Fanelli, E.; Pernice, P. Sol–gel Synthesis of ZnO Transparent Conductive Films: The Role of pH. Appl. Surf. Sci. 2014, 305, 194–202.

(15)

Liu, L.; Mei, Z.; Hou, Y.; Liang, H.; Azarov, A.; Venkatachalapathy, V.; Kuznetsov, A.; Du, X. Fluorine Doping: A Feasible Solution to Enhancing the Conductivity of High-Resistance Wide Bandgap Mg0.51Zn0.49O Active Components. Sci. Rep. 2015, 5 (316), 15516.

(16)

Liu, B.; Gu, M.; Liu, X.; Huang, S.; Ni, C. First-Principles Study of Fluorine-Doped Zinc Oxide. Appl. Phys. Lett. 2010, 97 (12), 122101.

(17)

Zhang, S. B.; Wei, S.-H.; Zunger, A. Intrinsic N -Type versus P -Type Doping Asymmetry and the Defect Physics of ZnO. Phys. Rev. B 2001, 63 (7), 75205.

(18)

Choi, Y.-J.; Park, H.-H. A Simple Approach to the Fabrication of Fluorine-Doped Zinc Oxide Thin Films by Atomic Layer Deposition at Low Temperatures and an Investigation into the Growth Mode. J. Mater. Chem. C 2014, 2 (1), 98–108.

(19)

Lee, H. B.; Ginting, R. T.; Tan, S. T.; Tan, C. H.; Alshanableh, A.; Oleiwi, H. F.; Yap, C. C.; Jumali, M. H. H.; Yahaya, M. Controlled Defects of Fluorine-Incorporated ZnO Nanorods for Photovoltaic ACS Paragon Plus Environment


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

Enhancement. Sci. Rep. 2016, 6, 32645. (20)

Zhang, J.; Liu, T.; Zhang, Y.; Zeng, W.; Pan, F.; Peng, X. Hydrothermal Synthesis and Growth Mechanisms of Different ZnO Nanostructures and Their Gas-Sensing Properties. J. Mater. Sci. Mater. Electron. 2015, 26 (3), 1347–1353.

(21)

Podrezova, L. V; Porro, S.; Cauda, V.; Fontana, M.; Cicero, G. Comparison between ZnO Nanowires Grown by Chemical Vapor Deposition and Hydrothermal Synthesis. Appl. Phys. A 2013, 113 (3), 623–632.

(22)

Wahab, R.; Kim, Y.-S.; Shin, H.-S. Fabrication, Characterization and Growth Mechanism of Heterostructured Zinc Oxide Nanostructures via Solution Method. Curr. Appl. Phys. 2011, 11 (3), 334–340.

(23)

Podrezova, L. V; Cauda, V.; Stassi, S.; Cicero, G.; Abdullin, K. A.; Alpysbaeva, B. E. Properties of ZnO Nanorods Grown by Hydrothermal Synthesis on Conductive Layers. 2014, 605 (8), 599–605.

(24)

Gao, Y.-F.; Miao, H.-Y.; Luo, H.-J.; Nagai, M.; Shyue, J.-J. Morphological and Crystallographic Transformation of ZnO in Solution. J. Phys. Chem. C 2008, 112 (5), 1498–1506.

(25)

Gao, X.; Li, X.; Gao, W.; Qiu, J.; Gan, X.; Wang, C.; Leng, X. Nanocrystalline/nanoporous ZnO Spheres{,} Hexapods and Disks Transformed from Zinc Fluorohydroxide{,} Their Self-Assembly and Patterned Growth. CrystEngComm 2011, 13 (14), 4741–4747.

(26)

Savoyant, A.; Alnoor, H.; Bertaina, S.; Nur, O.; Willander, M. EPR Investigation of Pure and CoDoped ZnO Oriented Nanocrystals. Nanotechnology 2017, 28 (3), 35705.

(27)

Sarma, B.; Sarma, B. K. Fabrication of Ag/ZnO Heterostructure and the Role of Surface Coverage of ZnO Microrods by Ag Nanoparticles on the Photophysical and Photocatalytic Properties of the MetalSemiconductor System. Appl. Surf. Sci. 2017, 410, 557–565.

(28)

Azad, A. K.; Han, J.; Zhang, W. Terahertz Dielectric Properties of High-Resistivity Single-Crystal ZnO. Appl. Phys. Lett. 2006, 88 (2), 1–3.

(29)

Ma, G.; Li, D.; Ma, H.; Shen, J.; Wu, C.; Ge, J.; Hu, S.; Dai, N. Carrier Concentration Dependence of Terahertz Transmission on Conducting ZnO Films. Appl. Phys. Lett. 2008, 93 (21), 21–23.

(30)

Han, J.; Zhang, W.; Chen, W.; Ray, S.; Zhang, J.; He, M.; Azad, A. K.; Zhu, Z. Terahertz Dielectric Properties and Low-Frequency Phonon Resonances of ZnO Nanostructures. J. Phys. Chem. C 2007, 111 (35), 13000–13006.

(31)

Joyce, H. J.; Boland, J. L.; Davies, C. L.; Baig, S.; Johnston, M. B. Electrical Properties of Semiconductor Nanowires: Insights Gained from Terahertz Conductivity Spectroscopy. Semicond. Sci. Technol. 2016, 31 (10), 1–21.

(32)

Baxter, J. B.; Schmuttenmaer, C. A.; V, Y. U.; Box, P. O.; Street, P. Conductivity of ZnO Nanowires, Nanoparticles, and Thin Films Using Time-Resolved terahertz spectroscopy. J. Phys. Chem. B 2006, 25229–25239.

(33)

Spierings, G. A. C. M. Wet Chemical Etching of Silicate Glasses in Hydrofluoric Acid Based Solutions. J. Mater. Sci. 1993, 28 (23), 6261–6273.

(34)

Banhegyi, G. Numerical Analysis of Complex Dielectric Mixture Formulae. Colloid Polym. Sci 1988, 266, 11–28.

(35)

Yordanov, N. D.; Gancheva, V.; Pelova, V. A. Studies on Some Materials Suitable for Use as Internal Standards in High Energy EPR Dosimetry. J. Radioanal. Nucl. Chem. 1999, 240 (2), 619–622.

(36)

Pupeza, I.; Wilk, R.; Koch, M. Highly Accurate Optical Material Parameter Determination with THz Time-Domain Spectroscopy. Opt. Express 2007, 15 (7), 4335–4350..

(37)

Scheller, M.; Jansen, C.; Koch, M. Analyzing Sub-100-Μm Samples with Transmission Terahertz Time Domain Spectroscopy. Opt. Commun. 2009, 282 (7), 1304–1306. ACS Paragon Plus Environment

Page 14 of 22

Page 15 of 22

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


(38)

Dorney, T. D.; Baraniuk, R. G.; Mittleman, D. M. Material Parameter Estimation with Terahertz Time-Domain Spectroscopy. J. Opt. Soc. Am. A. Opt. Image Sci. Vis. 2001, 18 (7), 1562–1571.

(39)

Duvillaret, L.; Garet, F.; Coutaz, J.-L. L. A Reliable Method for Extraction of Material Parameters in Terahertz Time-Domain Spectroscopy. IEEE J. Sel. Top. Quantum Electron. 1996, 2 (3), 739–746.

(40)

Angrisani, L.; Cavallo, G.; Liccardo, A.; Papari, G.; Andreone, A. THz Measurement Systems. In New Trends and Developments in metrology; Luigi Cocco, Ed.; InTech, 2016.

(41)

Papari, G. P.; Gargiulo, V.; Alfè, M.; Di Capua, R.; Pezzella, A.; Andreone, A. THz Spectroscopy on Graphene-like Materials for Bio-Compatible Devices. J. Appl. Phys. 2017, 121, 145107.

(42)

Smith, N. Classical Generalization of the Drude Formula for the Optical Conductivity. Phys. Rev. B 2001, 64 (15), 1–6.

(43)

Lovrinčić, R.; Pucci, A. Infrared Optical Properties of Chromium Nanoscale Films with a Phase Transition. Phys. Rev. B - Condens. Matter Mater. Phys. 2009, 80 (20), 1–6.

(44)

Muñoz-García, A. B.; Sannino, F.; Vitiello, G.; Pirozzi, D.; Minieri, L.; Aronne, A.; Pernice, P.; Pavone, M.; D’Errico, G. Origin and Electronic Features of Reactive Oxygen Species at Hybrid Zirconia-Acetylacetonate Interfaces. ACS Appl. Mater. Interfaces 2015, 7 (39), 21662–21667.

(45)

Vitiello, G.; Pezzella, A.; Calcagno, V.; Silvestri, B.; Raiola, L.; D’Errico, G.; Costantini, A.; Branda, F.; Luciani, G. 5,6-Dihydroxyindole-2-Carboxylic Acid–TiO 2 Charge Transfer Complexes in the Radical Polymerization of Melanogenic Precursor(s). J. Phys. Chem. C 2016, 120 (11), 6262–6268.

(46)

Vitiello, G.; Pezzella, A.; Zanfardino, A.; Silvestri, B.; Giudicianni, P.; Costantini, A.; Varcamonti, M.; Branda, F.; Luciani, G. Antimicrobial Activity of Eumelanin-Based Hybrids: The Role of TiO 2 in Modulating the Structure and Biological Performance. Mater. Sci. Eng. C 2017, 75, 454–462.

(47)

Chen, D.; Wang, Z.; Ren, T.; Ding, H.; Yao, W.; Zong, R.; Zhu, Y. Influence of Defects on the Photocatalytic Activity of ZnO. J. Phys. Chem. C. 2014, 118, 15300–15307..

(48)

Galland, D.; Herve, A. ESR Spectra of the Zinc Vacancy in ZnO. Phys. Lett. A 1970, 33 (1), 1–2.

(49)

Gonzalez-Hernandez, R.; Martinez, A. I.; Falcony, C.; Lopez, A. A.; Pech-Canul, M. I.; Hdz-Garcia, H. M. Study of the Properties of Undoped and Fluorine Doped Zinc Oxide Nanoparticles. Mater. Lett. 2010, 64 (13), 1493–1495.

(50)

Hichou, a El; Bougrine, a; Bubendorff, J. L.; Eboth , J.; Addou, M.; Troyon, M. Structural, Optical and Cathodoluminescence Characteristics of Sprayed Undoped and Fluorine-Doped ZnO Thin Films. Semicond. Sci. Technol. 2002, 17 (6), 607–613.

(51)

Petrik, N. G.; Taylor, D. P.; Orlando, T. M. Laser-Stimulated Luminescence of Yttria-Stabilized Cubic Zirconia Crystals. J. Appl. Phys. 1999, 85, 6770–6776.

(52)

Yousefi, R. Effects of Sn Atoms on Formation of ZnO Nanorings. CrystEngComm 2015, 17, 2698– 2704.

(53)

Withayachumnankul, W.; Naftaly, M. Fundamentals of Measurement in Terahertz Time-Domain Spectroscopy. J. Infrared, Millimeter, Terahertz Waves 2014, 35 (8), 610–637.

(54)

Titova, L. V.; Cocker, T. L.; Cooke, D. G.; Wang, X.; Meldrum, A.; Hegmann, F. A. Ultrafast Percolative Transport Dynamics in Silicon Nanocrystal Films. Phys. Rev. B - Condens. Matter Mater. Phys. 2011, 83 (8), 1–9.



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

Figure 1: Sketch of the experimental setup. The radiation is collimated and then focalized on each sample. The holder can be moved so that different pellets can be measured in the same experimental run.


Page 16 of 22

Page 17 of 22

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


F0

F1

2 µm

2 µm

F3

F5

2 µm

2 µm

Figure 2: SEM images show grains with a hexagonal pyramid shape typical of mesocrystals for bare ZnO (F0). At lower F at. %, ZnO grains are surrounded by smaller nanoparticles formed by the mesocrystals leaching (F1 and F3). The density of these nanoparticles decreases at higher fluorine concentration (F5).

1 µm

1 µm

1 µm

Figure 3: High Magnification SEM images of mesocrystals for bare ZnO (F0) and mesocrystals in the presence of smaller nanoparticles for F-doped (F3 and F5) samples.



a

(101) (100) (002) (110)

Intensity (a.u.)

(102)

(103)

F5

(112) (201)

F3

F1

F0

20

30

40

50

60

70

80

2θ

2θ θ

b

Intensity (a.u)

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 22

F5

ZnO 31

32

33

34

35

36

37

2θ

Figure 4. XRD pattern of bare and F-doped ZnO samples (a), an enlarged profile of XRD spectra of F0 and F5 samples (b)


Page 19 of 22

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


(VZn) g = 2.0024

F0

(Vo) g = 1.9568

F1

F3

F5

3300

3450

3600

3750

3900

B(G) Figure 5:EPR spectra for ZnO:F0 (black line), ZnO:F1 (blue line), ZnO:F3 (green line) and ZnO:F5 (red line) powders calcined at 350 °C for 5 hours.



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 20 of 22

Figure 6: Photoluminescence of (a) the bare ZnO and (b) 1%, (c) 3%, (d) 5% doped ZnO:F samples, excited by He-Cd laser emitting at 325 nm. (e) and (f): spectrum deconvolution for the bare and 5%-doped ZnO sample respectively.

(a)

(b)

Figure 7:UV peak shift (a) and UV/V peak ratio (b)as a function of Fluorine atoms percentage.


Page 21 of 22

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


Figure 8: (a) THz signals transmitted in the free space (black curve) and through the ZnO:F/KBr powders. Inset: expanded view of the THz signal for the different ZnO:F/KBr samples. (b) Transmittance of the ZnO:F/KBr samples under test. Samples with 0 at%, 1 at%, 3 at% and 5 at% of fluorine are represented with the red, brown, blue and magenta solid lines respectively:

Figure 9: (a) the real part of permittivity ( ) and b) conductivity (C)of ZnO:F samples are reported respectively. In the inset the imaginary part of permittivity .is displayed. Experimental data are plotted via circles, squares, rhombus and triangles corresponding to fluorine percentage of 0 at.%, 1 at.%, 3 at.% and 5 at.% respectively. Error bars for , and C are of the order of ±7%, ±1% and ±6% respectively. See the text for a wider explanation. Solid curves represent the fit to the standard Drude-Smith model: red, blue and magenta curves correspond to fluorine concentrations 0 at.%, 1 at.% and 5 at.% respectively.



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

Doping 0% 1% 3% 5%

49 49 50 50

10 13 13 11

C1 -0.70 -0.85 -0.83 -0.8

(S/cm) 6.4 2.4 2.9 4.0

Page 22 of 22

10 9.5 11 12

f

Tab.1: Parameters used for the best fit of the ZnO:F powders complex permittivity according to the Drude Smith model. The uncertainty of tabled parameters is of the order of 2%.

TOC Graphic


Morphological, Structural, and Charge Transfer Properties of F-Doped

Recommend Documents