ROL Y JERARQUIA DEL SISTEMA DE CENTROS POBLADOS

We measure the electronic transport of charge carriers in a bilayer graphene device, fabricated in a way schematically shown in Fig. 3.2a. We deploy a standard lock-in technique, with a low-frequency (13.33 Hz) excitation current I = 100 nAapplied to the bilayer, measuring the longitudinal voltage drop Uxx

along the device well outside the electrolyte-covered area in four-point probe geometry. In Fig. 3.2b, we show the sheet resistivity ρxx= W/L · Uxx/I thus

extracted during one lithiation/delithiation cycle. Both the width W = 3 µm of the bilayer as well as the distance L = 10 µm between neighboring probes used to measured Uxxare determined during patterning of the device. Lithiation (de-

lithiation) was driven at UG = 0.02V vs. Li/Li+ (UG = 2V vs. Li/Li+) with

the sample in a high-vacuum atmosphere (p ≤ 10−6_{mbar). Low-temperature}

experiments, the subject of chapter 4, were conducted between lithiation and delithiation, accounting for the discontinuity in the time axis.

According to Fig. 3.2b, the sheet resistivity ρxx of bilayer graphene ex-

periences drastic changes during lithiation and delithiation, although ρxx is

measured well outside the electrolyte-covered area of the bilayer. These chan- ges are caused by a charge transfer on the order of e, the elementary charge of an electron, from each intercalating Li-ion to the electronic states of bilayer graphene (see Sec. 2.1.1). In terms of the simplifying rigid band model, this charge transfer shifts the Fermi level F while leaving the dispersion of the

host's electronic states unchanged. Consequently, when Li-ions enter bilayer graphene during lithiation, F shifts further up into the conduction band as the

systems gets more and more electron doped. In the reverse case, F drops du-

Figure 3.2: (a) Schematic device design as in Fig. 3.1. (b) Longitudinal sheet re- sistivity ρxxof an initially p-doped bilayer graphene device measured in situ during

one electrochemical lithiation/delithiation cycle. Insets illustrate the position of the Fermi level F at three dierent times.

transfer. This is illustrated in the insets to Fig. 3.2b with the grey lines repre- senting the approximately isotropic dispersion of the lower-lying π-electronic bands around K(0)_{. For this initially highly p-doped sample (due to a high}

impurity density nimp≈ 1.3 · 1013cm−2), we start with F well in the valence

band. During lithiation, F slowly shifts towards the conduction band, causing

a maximum sheet resistivity ρCNP

xx ≈ 17.3 kΩ when crossing the charge neu-

trality point (CNP). ρxx then drops as F rises further with continued inux

of Li-ions providing more electrons to the bilayer electronic states. The sub- sequent delithiation step, here performed after a few days with the sample at temperatures as low as T ≈ 1.5 K, begins with F still high in the conduction

band. As Li-ions leave bilayer graphene during delithiation, F shifts towards

the valence band and we see a concomitant increase of ρxx up to about the

same maximum value of ρCNP

xx at the charge neutrality point. Due to the high

level of intrinsic p-doping, F continues to go deeper into the valence band as

delithiation proceeds, paralleled with a corresponding decrease of ρxx. At the

end of this lithiation/delithiation cycle, ρxxattains just about the value it had

at its beginning, suggesting a high degree of reversibility of the process. The behavior of ρxxmeasured well outside the electrolyte-covered area sup-

ports the assumption that during lithiation Li-ions intercalate into bilayer grap- hene and diuse throughout the latter in order to minimize concentration gra- dients. Due to charge transfer, the electronic states in bilayer graphene get increasingly populated as the Li-concentration rises, thereby shifting F and

causing a related change in ρxx akin to its backgate-dependence35 discussed

Figure 3.3: (a) Sheet resistivity ρxx,i measured in situ during four successive lithi-

ation/delithiation cycles of an initially p-doped bilayer graphene device using three neighboring contact pairs i, with i = 1 (i = 3) closest (furthest) to the electrolyte. Fig. 3.2b is an excerpt of the shown data for ρxx,3. As in Fig. 3.2b, insets illustrate the

position of the Fermi level F at dierent times. (b) Scanning electron micrograph

of the bilayer graphene device after cycling. Ti contact pairs for measurements of longitudinal voltage drops Uxx,iused to extract ρxx,iare indicated in the same color

as respective data in (a).

voltage applied to the backgate, here the precise time-evolution of ρxx during

lithiation (respectively delithiation) likely depends on the Li-diusion dyna- mics within the electrolyte, the solid electrolyte interphase (SEI) formed at the bilayer graphene/electrolyte interface as well as the bilayer itself. Microscopic details such as wrinkles, folds, and atomic scale defects in the latter may act as barriers to diusion. Furthermore, with side-reactions imposing a non-zero degree of irreversibility, each lithiation cycle begins under slightly altered con- ditions the impact of which on the lithiation kinetics may turn out more drastic in the given case of a single-crystalline electrode than in the conventional case of composite electrodes.*

Fig. 3.3a shows ρxx,i during four successive lithiation/delithiation cycles of

*_{In a macroscopic galvanic cell with composite electrodes, local changes with a critical}

impact on the local Li diusion kinetics inict only a minor change on the overall cycling behavior of the cell.

a bilayer graphene device (shown in panel b) measured at three neighboring positions i with i = 1 (i = 3) closest (furthest) to the electrolyte. Fig. 3.2b is in fact an excerpt of the shown data for ρxx,3. While the three traces of

ρxx,i(t) nearly overlap, nite dierences between them could indicate a spa-

tial nonuniformity of the sample (e.g., due to an inhomogeneous distribution of impurities) or of the Li diusion process (the subject of chapter 6). They could further stem from irreversible processes during lithiation. For example, ρxx,1(t ≥ 144 h)never drops back to ρxx,1(0)during delithiation, which could

be due to a wrinkle across the bilayer within the probed area (indicated in the scanning electron micrograph in Fig. 3.3b) at which immobilized Li reaction products are found to agglomerate with time. Such decorations are subject to more in-depth discussion in chapter 5. More striking however is the overall dierence between ρxx,i(t)for successive lithiation/delithiation cycles. Despite

polarization durations being unequally long, ρxx,i(t) during delithiation steps

show a higher degree of resemblance than during lithiation steps. The rate limiting factor is thus mainly determined by the diusion kinetics in the elec- trolyte and/or the SEI, which are susceptible to modications due to cycling (as SEI formation continues and its microstructure changes) or due to ageing. Thus, the second lithiation in Fig. 3.3a appears hampered with ρxx,i(t)uctu-

ating at values above 5 kΩ which is not related to problems in the bilayer, but rather to limitations on the Li-inux. Note that in addition to what was said before, the bilayer here is connected to the electrolyte via a natural connection to a bulk graphite single crystal, see Fig. 3.3b. Possible intercalant ordering in this bulk part might further impede the migration of Li into the bilayer. If Li-ions are provided quickly enough, as during the fourth lithiation in Fig. 3.3a, ρxx,i(t)changes equally fast at all three contact pairs, indicating a high intrin-

sic Li diusion coecient in bilayer graphene. Its quantitative determination is discussed in chapter 6.

The continuous change of ρxx in bilayer graphene during lithiation cycles

has not been experimentally demonstrated before. However, it is known that the intercalation of bulk graphite lends the resulting compound a metallic character, in contrast with the semi-metallic one of graphite itself.5 _{A re-}

duction of the anisotropy in the electronic conductivity σa/σc from 3000 in

HOPG to 14 in LiC6 has been reported, with an approximately 10-fold in-

crease in the in-plane conductivity from σa ≈ 2.5 · 104Ω−1cm−1 in HOPG to

σa ≈ 2.5 · 105Ω−1cm−1 in LiC6, nearing the room-temperature conductivity

of copper σCu ≈ 6 · 105Ω−1cm−1.2 Here, σc is the out-of-plane conductivity

in graphite (parallel to the c-axis). Also in bulk graphite, the increase in the in-plane conductivity is generally understood as being due to charge transfer

from the intercalate layer to the graphene planes.2 _{An increase in σa upon Li}

intercalation has been demonstrated also in few-layer graphene samples with a thickness down to ve layers. In this work, Bao et al.122 _{report maximum}

room-temperature values of σmax

a ≈ 11 mSper layer in LiC6, claimed to be one

third of the intrinsic limit set by electron-acoustic phonon scattering. In the case of our device discussed in this section, the minimum room-temperature sheet resistivity obtained after lithiation ρxx ≈ 700 Ω yields σa ≈ 0.7 mS per

layer indicating most likely a yet incomplete intercalation. It should be no- ted however, that the dierence between the conductivity in a pristine bilayer graphene ake with respect to a fully Li intercalated one is likely much higher than the factor of ∼ 10 reported for bulk graphite2_{, as Bao et al.}122 _report

values up to about 50 for few-layer samples and we nd a factor of about 30 when comparing the value at charge neutrality σCNP

a ≈ 0.025 mSper layer to

our minimum after lithiation σa≈ 0.7 mSper layer. If indeed at full Li-content

σ_amax≈ 11 mSper layer, the increase in conductivity with respect to the pris- tine state would exceed a factor of 400. To our knowledge, such a high value would be unprecedented for Li-intercalated graphitic carbon.